Allow for custom datatype names in votable

This should allow end users to workaround VOTable files that contain
invalid datatypes.

Also adds two datatypes seen in Vizier tables: "unsignedInt" and "unsignedShort".
Update docs about datatype mappings.
Add changes entry.
Fix link to h5py package in docs/doctrings

Old url was http://alfven.org/wp/hdf5-for-python, new is http://h5py.org
Fix line lengths for PEP8
Overwrite individual tables in HDF5 file

Add the ability to overwrite a single table within an HDF5 file without
affecting other files if user passes both `append=True` and
`overwrite=True` to the `Table.write()` method. (Using `overwrite=True`
alone deletes the HDF5 file, and any other datasets inside it, while
`append=True` alone can only add new datasets to an HDF5 file.)
Adds `test_read_write_existing_append_overwrite` function to test this
combination of arguments to `write()`.

Closes #2624
Document append+overwrite arg combo for HDF5

See #2624
Don't rewrite the config file on every import.
Add docs for issue #2762
Add check for np.ma.nomask in NDData mask setter.

Closes #2784
Added changelog entry for #2784
Add regression test for #2784
Merge pull request #2796 from AnthonyHorton/bugfix-for-issue-2784

Add check for np.ma.nomask in NDData mask setter
Fixed support for staticmethods in deprecated() and fixed an issue where the full deprecation message was not being displayed
Added changelog entry
Fixed HTML tests in the case that lxml is not available as a backend
Updated bundled six module to latest version (1.7.3)
Merge pull request #2813 from amras1/handle-no-lxml

Fixed HTML tests in the case that lxml is not available as a backend
Merge pull request #2627 from jonathansick/overwrite-hdf5-dataset

Overwrite single HDF5 dataset without overwriting file
Updated Cone Search URLs for Vizier because they changed in the STScI registry.