Autodoc the plugins

Replaces all the TODO plugin docs with autodoc of the module. Plugin
developers can now use pydoc to document their plugins.

For the docs to be buildable freeseer source code must exist in the
same root directory as freeseer-docs.

Freeseer/freeseer#365
Signed-off-by: Thanh Ha <thanh.ha@alumni.carleton.ca>