Fix #27: Add in a changelog.

Contains development history up until now, plus changes planned for next
release.
Release 0.2.

* Abstract out tracking code and have first shot at implementing a Kalman filter based tracker
* Implement StormtracksResultsManager that can be used for saving/loading results to disk
* Improve pyro_cluster code to the point where it is usable (speedups of 10x using 15 UCL computers)
* Add logging to pyro_cluster code to aid debugging
* Handle lons/lats consistently - always use -180 to 180 in code
* Add some functional/bug tests
* PEP8 test all code in tests
* Document most classes/functions (and add a coverage check in documentation)
* Make names more consistent in code
* Fix some bugs