(fix) Constant time HMAC comparison

Fixes #22.
Merge pull request #28 from veeti/fix/fixed-time-eq

(fix) Constant time HMAC comparison
(cleanup) Switched entirely to using cargo for builds.
(fix) Migrated to rust-url.
(fix) Conformed to iron changes.
(test) Updated tests.
(docs) Updated examples.
(fix) Stop using fixed_time_eq to avoid segfault.
(fix) Passed tests.
(docs) Updated README.