add no_prealloc

[oxinabox]

Closes #3

[baggepinnen]

I tried the following

AutoPreallocation.@no_prealloc Xi = Γi \  Zi

but it gave me

UndefVarError: Xi not defined
#n4sid#63(::Bool, ::Int64, ::Nothing, ::typeof(n4sid), ::Array{Float64,1}, ::Array{Float64,1}, ::Int64) at subspace.jl:105
n4sid(::Array{Float64,1}, ::Array{Float64,1}, ::Int64) at subspace.jl:53
top-level scope at autoprealloc.jl:4

If I do

Xi = AutoPreallocation.@no_prealloc(Γi \  Zi)

then it works!