Two new features:
 - Add support for includes. load() now takes an optional include_dirs argument that specifies relative paths.
 - Add support for service "extends".
Fix lint errors.
PY3 compat.
Slightly controversial: include the enum type in I32 definitions coming from enums.

This makes it a lot easier to build reflection-like tools, e.g. converting JSON to Thrift that might use strings for enums.
Still bad at lint.
Merge pull request #37 from mikekap/develop

Add support for includes & service extends