Sentence and DOM moved, other fixes

Various little bug fixes, along with: Sentence and DOM have been moved
to their own files, but in such a way that other modules cannot tell;
Math now bails out to wolfram alpha in case of failure; open_URL gained
linux support (sort of); Some docstring updates; There is now a very
basic Sentence Diagram included with the sentence object —to view it,
simply print Sentence.diagram.