[WIP] Adding file upload resource and multipart post requests

This PR adds the ability to post multipart form data from any resource as well as send requests to a different api url (specifically `uploads.stripe.com` for file uploads). The majority of the multipart work is done in the `MultipartProcessor` class which takes an http connection. 
Can you implement hashCode && equals on the `FileUpload` class, then:

1. Remove URL and any other dynamic fields - check that they match a regex.
2. call `assertEquals` on the two `FileUpload` objects