Make dataset, group, and versionedhdf5file reprs more informative

[peytondmurray]

Currently, versioned-hdf5 uses reprs that pretty closely follow the h5py reprs for the analogous data types. There provide no useful information about the data hierarchy that lives under the object in question:

(Pdb++) pp sv
<InMemoryGroup "/_version_data/versions/r02" (0 members)>

I'd really like Group objects to actually say the name of their keys, and provide some hint about the type of object stored under each key.

This isn't a major problem, but it is annoying to debug because you always have to call pp list(obj) or pp obj.keys() to find out what data is stored in a group or file.

[peytondmurray]

closing for now as completed with #319.