Explicit exports for the xylem.sources package.
Add documentation about used coding style.
Added some API documentation.

 - Use apidoc to generate full API doc (for now e.g. to check syntax in
   docstrings).
 - Once more of the API is fleshed out the documentation should be properly laid
   out and structured.
Merge pull request #6 from catkin/api-doc

Stub for API doc and a small section on coding style in the docs.