mmap arrays out of files. API still in flux.
Integrate mmap with JLD

Move mmap functionality to readmmap function in plain.jl, and add
mmaparrays named argument for opening JLD files.
Use default argument
readmmap(HDF5Dataset), iscontiguous(), ismmappable()
Merge pull request #22 from simonster/mmap

RFC: Add option to mmap arrays from JLD files