Multipart Form Upload Support

To close #15.

Unfortunately Python's http libraries don't include multipart form support, so it's necessary to do quite a lot to get this working.

This PR is similar to @akirasan's code in #15, but also handles OAuth signing of parameters.

I've also updated the commandline tool to support the using the "@" syntax to specify a file to upload:

    openphoto -p -X POST -H akirasan.trovebox.com -e /photo/upload.json -F 'photo=@/mnt/ksi2t/scripts/DSC_8422.JPG' -F 'tags=test'