Replace to_str() by to_string()

Also had to rework `from_rfc_compliant` a bit due to the liburl API change.
Fix Travis CI build.
Merge pull request #20 from michaelsproul/travis-and-to-str

Fix Travis CI and replace .to_str() by .to_string()
(fix) Use HMAC for signatures

Using a plain SHA-256 hash for message authentication is vulnerable to
length extension attacks. Use the secure HMAC construct from rust-crypto.