Fixes/adjustments from review.

This change implements most of the formatting/compatibility fixes suggested in
https://github.com/shazow/urllib3/pull/220.

1) Renamed `RequesteParam` to `RequestField` 2) Put __str__ method on a single
line 3) Switched `iteritems` calls to `items` to fix Python 3 compatibility 4)
Changed `type` calls to `isinstance`

I still haven't moved away from method chaining or adjusting the constructor.
We still need to discuss what the right approach is and if we still need
`make_multipart` after adjusting `ParamField.__init__` to take arbitrary
arguments.