Make typed_ast mypy's default and only parser

[ddfisher]

Currently, typed_ast is mypy's preferred parser (as it's much faster, more correct, and supports features like multiline type comments), but it is not yet mypy's default parser. The main blocker to this is Windows support.

Here are the high-level items remaining:

• make typed_ast compile and run correctly on Windows
• autobuild Windows wheels with AppVeyor
• [optional] autobuild OS X wheels in TravisCI
• update tests to account for minor line differences between typed_ast and the current parser
• make typed_ast the default parser
• remove old parser

[ddfisher]

After this is complete, we can start converting mypy to use typed_ast's ast (which is very similar to the official python ast) node by node (which will further help perf).

[ddfisher]

typed_ast code now available at https://github.com/dropbox/typed_ast