Moved all code into "baon" top-level package

This is the first step in making BAON a proper Python application package.
Added unit tests for simple text functions (except title())
Bugfixed inparas/inbraces/incurlies functions

Unit tests revealed that they would not correctly handle some cases.
Improved to_title_case() and added unittest for it
Started switching to new match execution model

In the new model, matches yield a sequence of possible "solutions", i.e. possible contexts after the execution of the match, where the text has been consumed and placed in 'matched_text', the 'position' has been advanced, etc.

Note that this completely breaks the matches codebase. In the following diffs, the matches will be converted to the new system one by one.
Switched ElementaryPatternMatch to new model
Bugfixed LiteralMatch and added unit tests for it
Bugfixed RegexMatch
Added unit tests for RegexMatch
Bugfixed FormatMatch
Added unit tests for FormatMatch