Add HTTP Digest Authentication support

[mksh]

This contains changes from https://github.com/twisted/treq/pull/131 ,

with review comments

https://github.com/twisted/treq/pull/131#discussion_r78300154 https://github.com/twisted/treq/pull/131#discussion_r78300154 https://github.com/twisted/treq/pull/131#discussion_r78301644

addressed,

e.g.

• the digest authentication parameter cache was made bound to HTTPDigestAuth object;
• low-level header build machinery was moved to the said class' methods;
• unit test cases were added for HTTPDigestAuth, improving auth module coverage.