Update Makefile.
Set-up testing configs.
Add signature tests based on Eran Hammer's guide.

Found at:
https://web.archive.org/web/20131222062830/http://nouncer.com/oauth/signature.html
Add credentials constructor for testing purposes.
Add signature key and text accessors, for testing.
Fix signature digest: must be encoded.
Fix parameter string construction in sig base string.
Add option to exclude query at URI serialization.
Fix URI query parsing: remove "?".
Fix base string URI: exclude query.
Handle optional protocol parameter: "oauth_version".