Changes needed to precompile

The changes needed to get this to work with package precompilation were impressively minor. The only surprise was the need to call `rehash`, and I'm not sure why that's necessary. (I also don't understand why the other dictionary, `hdf5_prop_get_set`, does not seem to need rehashing.) CC @JeffBezanson, @vtjnash.

Currently, the user needs to call HDF5.init() in their .juliarc.jl script or before using HDF5 functions.