Fix indexing NaT scalars
Read a gzipped netcdf file
Add test
Reading gzip files in Python 2.6 not supported
Merge pull request #239 from lakshmanok/master

Read a gzipped netcdf file
Merge pull request #240 from shoyer/fix-index-NaT

Fix indexing NaT scalars
Restructured Backends to make CF convention application more
consistent.

Amongst other things this includes:
- EncodedDataStores which can wrap other stores and allow
  for modular encoding/decoding.
- Trivial indices ds['x'] = ('x', np.arange(10)) are no longer
  stored on disk and are only created when accessed.
- AbstractDataStore API change.  Shouldn't effect external users.
- missing_value attributes now function like _FillValue

All current tests are passing (though it could use more new ones).

Post rebase notes (shoyer, Oct 2, 2014):
Most tests are passing, though a couple are broken:
- test_roundtrip_mask_and_scale (because this change needs a fix to not
  break the current API)
- test_roundtrip_strings_with_fill_value on TestCFEncodedDataStore
  (I don't entirely understand why, let's come back to it later)