Multipart request improvements.

* Added fileName() to TypedOutput so non-File parts can be named.
* Support Content-Length in multipart requests.
* Serialize parts to byte[]s so length is available.
Merge pull request #183 from codebutler/master

Issues with new multi-part implementation
Reset thread name to idle when not in use.

Closes #188.
Merge pull request #190 from square/jw/thread-name-reset

Reset thread name to idle when not in use.
Exposes the static RetrofitError construction methods

Allows `RetrofitError`s to be thrown from code outside of Retrofit
Merge pull request #192 from imminent/patch-1

Exposes the static RetrofitError construction methods