hash function is fragile/broken [InexactError]

[rickhg12hs]

Wondering if the conversion functions/constructors in the DataArray hash function should perhaps be casts/reinterprets.

Fixing this will also probably fix #146.

julia> versioninfo()                                                                                                                                         
Julia Version 0.4.0-dev+4436
Commit 237cdab (2015-04-22 06:02 UTC)
Platform Info:
  System: Linux (x86_64-linux-gnu)
  CPU: AMD Opteron 63xx class CPU
  WORD_SIZE: 64
  BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY Piledriver)
  LAPACK: libopenblas
  LIBM: libopenlibm
  LLVM: libLLVM-3.3

julia> Pkg.status("DataArrays")                                                                                                                              
 - DataArrays                    0.2.13             master

julia> hash(DataArray(["Julia", "Rocks", "Dude!"]))                                                                                                          
ERROR: InexactError()
 in hash at /projects/02919452-9afb-425c-988d-f3b3cc5dd31a/.julia/v0.4/DataArrays/src/dataarray.jl:677

[cstjean]

Another InexactError: hash(@data [-1]) # either -1 or -1.0 fails