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.
Don't rewrite the config file on every import.
Updated bundled six module to latest version (1.7.3)
Added note to changelog
Made 1.7.3 the minimum six version
For CSS formatting of "Other Parameters" section.
Merge pull request #2805 from mdboom/config/always-write

Astropy 0.4 rewriting astropy.cfg all the time, leading to race conditions
Catch exceptions in this loop; this makes me wonder now how much time it takes to display a warning.  I wonder if we shouldn't be caching results here.  But this fixes #2671 at least.
Merge pull request #2814 from amras1/update-six-version

Updated bundled six module to latest version (1.7.3)
Merge pull request #2675 from mdboom/votable/custom-datatype-mapping

VOTable: mask out invalid data types?
Merge pull request #2580 from embray/modeling/model-refactor-1

Model refactoring stage 1: "evaluate" normalization
Merge pull request #2671 from embray/issue-2671

ImportErrors possible within AstropyLogger
Update astropy_helpers to the commit that will be tagged v0.4.1
Added changelog entry for this
Merge pull request #2825 from embray/astropy-helpers-update

Update astropy_helpers to the commit that will be tagged v0.4.1
Update ah_bootstrap.py from soon-to-be astropy-helpers v0.4.1
Tweaks to enable importing/running astropy out of a source checkout on Python 3
Merge pull request #2827 from embray/astropy-helpers-update

Update ah_bootstrap.py from soon-to-be astropy-helpers v0.4.1