Add from __future__ import unicode_literals to a few files.

Still need to do some py3k-only ones.

- 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.