Added multipart support for upload/download of files.

Delete is not supported.
Added more content types for upload/download
Code cleanup.

Introduce BinaryCallback, revert back Callback.onSuccess. Add unit-test
for downloading binary data.

Clean up FORM_MULTIPART related code.

Upgrade android-async-http to 1.4.4.
Merge pull request #21 from strongloop/feature/binary-payload

Multipart support for upload/download of files