Mostly by @gdooms, with tweaks.

- Stop parsing new templates if the template depth gets above
  MAX_DEPTH (40) or if we've already tried to parse over MAX_CYCLES
  (100,000) templates.
- Add two tests to ensure recursion works somewhat correctly.
- Fix parsing the string "{{" with the Python tokenizer; add a test.

- Newlines are now allowed in template param names.
- Changes also affect handling of arguments like {{{foo}}}.
- Update unit tests: remove some unnecessary ones, and add some to cover the changes.
- Update StringMixIn tests to actually work for some of the methods.
- Update copyright notices for the C extensions.