Support file attachments on HTTPSFuture

Summary: @zeeg wants this for some build stuff and we'd like it in general to make Mailgun work better. It's a bit messy because of how cURL works. See pages of comments inline.

Test Plan:

• Added tests for the parser stuff.
• Made various requests with and without file attachments and saw them encode data as expected.
• Made some dangerous requests (with "@") and saw them fail.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley, zeeg

Differential Revision: https://secure.phabricator.com/D8831