provide a friendlier API for multipart forms.

The super-generic `MultipartBuilder` API is pretty nice, but probably the most common case is building multipart forms. This patch provides a couple of convenience methods preventing the caller from having to write `Content-Disposition: form-data` all the time.

Open questions:
* should the `addFormPart(String name, String value)` overload provided a default content-type, presumably `text/plain; charset=utf-8`? [RFC 2388](http://www.ietf.org/rfc/rfc2388.txt) implies that the answer is yes.
* should we bother to do the quoted-string encoding if the caller includes "special characters" in the field name or filename parameter?

Also note that I can't get the tests to pass even before my changes; a quick review seems to indicate that I didn't make anything *more* broken with these changes, but apologies if I'm wrong here.
hm, this is the "generic" version of the method which can do anything, not just files; as you can see, the simple (String,String) adder is just a tiny wrapper around this one.

renaming them both to `addFormDataPart` for now; let me know if you have a better idea. Should we do something (beyond throwing an exception) to make it more obvious that the `filename` parameter can be null but the `name` parameter can't? I don't see any such convention elsewhere in this project but please let me know if I'm missing something.