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
Initial refactoring so that every, or almost every model class shares a
generic __call__, with all the machinery for evaluating a given model in
a .evaluate() method that every model is required to have--preferably a
staticmethod or a classmethod so that it can implemented easily
independtly of the model class it belongs to.

The new system is not yet implemented for composite models or for
polynomial models, which will require some further refactoring.
Completed initial conversion of all models to use the new interface of implementing all its core calculation in an evaluate method.  This includes prepare_input and prepare_output methods that can be overridden by subclasses and used also by fitters.  The exact semantics of prepare_input and prepare_output I am not quite happy with yet--currently they do what is necessary to preserve the existing input handling, which I intend to rework.  Finally, Model.param_sets is no longer an ndarray, but just a normal list (albeit of ndarrays).  This was the only way I could get it to work consistently with multi-dimensional parameter values.  I plan to replace Model.param_sets with something else later.
Really, Model.evaluate should be an abstractmethod
Fix bad reference in the docstring; hopefully this will fix the doc build
Fixed up the new model examples to reflect the new implementation (no mention of process_input/output but I think that's a pretty advanced thing most models won't need to touch).  Fixed, I think, the documentation references that were failing
Remove dangling (and misspelled) reference to format_input
Remove stray __call__ implementation; this is no longer needed here
Fixed indentation in code examples
Add somewhat more graceful deprecation for the eval -> evaluate changeover
Squelch a warning in the plotting code here.  It's harmless, but annoying to have whenever building the docs.
Address a few minor typos and other issues pointed out by @cdeil
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.