h5t methods

[bnlawrence]

I've had to create, for the moment, an h5t emulation layer, with three methods which really have little place in pyfive as it currently stands. It might be that we do need this layer for complete vlen str compatibility with H5Py (insofar as a vlen string should return the same dtype, it doesn't currently).

This is all part of h5netcdf support (#7).

The issue here is, do I leave these here, will we need them (e.g. if we do go to enumeration support and/or complete vlen compatability?), or can we move them elsewhere (e.g. into h5netcdf) because they're not really part of the necessary API?

[bnlawrence]

These exist, and utilise, because the H5Py vlen and enumerations inherit a class called DataType which has a metadata method ...