Gradients, second derivatives and hessians at pi not working

[islent]

You might need some fixes with README 😄
Related issues #40 #52 : float(pi) could fix this:

julia> using Calculus

julia> h1 = second_derivative(sin)
#6 (generic function with 1 method)

julia> h1(0.0)
0.0

julia> h1(1.0)
-0.841471649579559

julia> h1(pi)
ERROR: MethodError: no method matching eps(::Type{Irrational{:π}})
Closest candidates are:
  eps(::Dates.Time) at D:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.3\Dates\src\types.jl:387
  eps(::Dates.Date) at D:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.3\Dates\src\types.jl:386
  eps(::Dates.DateTime) at D:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.3\Dates\src\types.jl:385
  ...
Stacktrace:
 [1] finite_difference(::Calculus.var"#1#3"{typeof(sin),Symbol}, ::Irrational{:π}, ::Symbol) at C:\Users\islent\.julia\packages\Calculus\mbqhh\src\finite_difference.jl:27lus\mbqhh\src\finite_difference.jl:27
 [2] finite_difference_hessian(::Function, ::Function, ::Irrational{:π}, ::Symbol) at C:\Users\islent\.julia\packages\Calculus\mbqhh\ckages\Calculus\mbqhh\src\derivative.jl:27src\finite_difference.jl:224
 [3] (::Calculus.var"#6#8"{typeof(sin),Calculus.var"#1#3"{typeof(sin),Symbol},Symbol})(::Irrational{:π}) at C:\Users\islent\.julia\packages\Calculus\mbqhh\src\derivative.jl:27
 [4] top-level scope at REPL[5]:1

julia> h1(float(pi))
-1.3553766145945872e-7

julia> g2 = Calculus.gradient(x -> sin(x[1]) + cos(x[2]))
#2 (generic function with 1 method)

julia> g2([0.0, 0.0])
2-element Array{Float64,1}:
 0.9999999999868854
 0.0

julia> g2([1.0, 1.0])
2-element Array{Float64,1}:
  0.5403023058631036
 -0.8414709847974693

julia> g2([pi, pi])
ERROR: MethodError: no method matching Irrational{:π}(::Int64)
Closest candidates are:
  Irrational{:π}(::T) where T<:Number at boot.jl:718
  Irrational{:π}() where sym at irrationals.jl:18
  Irrational{:π}(::Complex) where T<:Real at complex.jl:37
  ...
Stacktrace:
 [1] convert(::Type{Irrational{:π}}, ::Int64) at .\number.jl:7
 [2] zero(::Type{Irrational{:π}}) at .\number.jl:266
 [3] float(::Array{Irrational{:π},1}) at .\float.jl:896
 [4] (::Calculus.var"#2#4"{var"#3#4",Symbol})(::Array{Irrational{:π},1}) at C:\Users\islent\.julia\packages\Calculus\mbqhh\src\derivative.jl:5
 [5] top-level scope at REPL[9]:1

julia> h2 = hessian(x -> sin(x[1]) + cos(x[2]))
#7 (generic function with 1 method)

julia> h2([0.0, 0.0])
2×2 Array{Float64,2}:
 -7.5693e-7   0.0     
  0.0        -0.999999

julia> h2([1.0, 1.0])
2×2 Array{Float64,2}:
 -0.841472   0.0
  0.0       -0.540303

julia> h2([pi, pi])
ERROR: MethodError: no method matching Irrational{:π}(::Int64)
Closest candidates are:
  Irrational{:π}(::T) where T<:Number at boot.jl:718
  Irrational{:π}() where sym at irrationals.jl:18
  Irrational{:π}(::Complex) where T<:Real at complex.jl:37
  ...
Stacktrace:
 [1] convert(::Type{Irrational{:π}}, ::Int64) at .\number.jl:7
 [2] zero(::Type{Irrational{:π}}) at .\number.jl:266
 [3] float(::Array{Irrational{:π},1}) at .\float.jl:896
 [4] (::Calculus.var"#2#4"{var"#5#6",Symbol})(::Array{Irrational{:π},1}) at C:\Users\islent\.julia\packages\Calculus\mbqhh\src\derivative.jl:5
 [5] finite_difference_jacobian(::Calculus.var"#2#4"{var"#5#6",Symbol}, ::Array{Irrational{:π},1}, ::Symbol) at C:\Users\islent\.julia\packages\Calculus\mbqhh\src\finite_difference.jl:197
 [6] finite_difference_hessian(::Function, ::Function, ::Array{Irrational{:π},1}, ::Symbol) at C:\Users\islent\.julia\packages\Calculus\mbqhh\src\finite_difference.jl:284
 [7] (::Calculus.var"#7#9"{var"#5#6",Calculus.var"#2#4"{var"#5#6",Symbol},Symbol})(::Array{Irrational{:π},1}) at C:\Users\islent\.julia\packages\Calculus\mbqhh\src\derivative.jl:29
 [8] top-level scope at REPL[15]:1