Improved conformance to PEP 8 guidelines.

A number of rules are ignored for now (see the Travis configuration file). This PR does:

- Remove trailing whitespace ;
- Make indentation consistent (no more mixes of tabs and spaces) ;
- Make sure there is proper whitespace around modulo and bitwise operators ;
- Replaces used of ``has_key()`` with ``in`` (see http://stackoverflow.com/questions/1323410/has-key-or-in).

Travis is still failing because there's some work left to be done (see https://travis-ci.org/PierreLvx/FS2XPlane). All in due time :)