Reorganize package, generate new documentation

Reorganize the RiveScript package. The bulk of the code has been moved
to rivescript/rivescript.py, and __init__.py mostly just contains the
meta information. This is because epydocs was getting confused by all
the imports in __init__.py and generating documentation for third party
modules. Usage of the rivescript module has not changed, you can still
do "from rivescript import RiveScript"