Make InMemoryGroup provide more informative reprs

[peytondmurray]

This PR makes InMemoryGroup reprs more informative, closing #316.

h5py.Group reprs don't by default show anything about their data hierarchy. This change makes InMemoryGroup reprs show their child objects as part of their repr, making them substantially more useful. For example, in the test written below, repr(premade_group) returns:

<InMemoryGroup "/group" (3 members)>
  <InMemoryArrayDataset "foo": shape (6,), type "<f8">
  <InMemoryArrayDataset "bar": shape (6,), type "<f8">
  <InMemoryArrayDataset "baz": shape (6,), type "<f8">

Later on, we might consider replacing InMemoryDataset.keys() and InMemoryDataset.values(), and InMemoryDataset.items() to return more informative string representations as well.