added support for reading static files from package

cf. discussion at
https://github.com/FND/tiddlywebplugins.bfw/pull/19#issuecomment-21953283

overloading existing configuration parameter for backwards compatibility

I opted not to allow for any kind of complex configuration or cascade of
different source directories and URIs since there's no tangible use case
for that yet (i.e. YAGNI)
Merge pull request #1 from FND/pkg_resources

added support for reading static files from package