Add multi-part message support for update_with_media endpoint

This pull request adds a feature that allows for multi-part message support, specifically for the update_with_media endpoint. I've manually built the body of this message rather than using another module, as the modules I reviewed did not support the net/http object that we use for OAuth headers.

Feel free to ping me with any questions, concerns, etc. Not sure if it would be possible, but would love to get this merged later this week so we can publish new documentation on dev.twitter.com that points to using this feature.

Jacob
jpetrie@twitter.com
@jaakkosf