Added tests + documentation for `RequestFields`

Made last changes requested in https://github.com/shazow/urllib3/pull/220.
Namely, 1) added tests cases to bring to 100% test coverage, 2) added
documentation and cleaned up docstrings.

I cleaned up parts of the code that were actually impossible to run -- they
were artifacts of prototyping that I never thought to remove.

I took the liberty of renaming `params.py` to `fields.py` for obvious reasons.
=)