Add HTTP Signature support.

Add options, docs, and tests for HTTP Signature Scheme[1] support using
Joyent's library[2].

[1] https://github.com/joyent/node-http-signature/blob/master/http_signing.md
[2] https://github.com/joyent/node-http-signature
Merge branch 'master' of git://github.com/mikeal/request

Conflicts:
	README.md
Merge branch 'master' of git://github.com/mikeal/request
Merge pull request #510 from digitalbazaar/master

Add HTTP Signature support.