accept extra attributes of user

keystone supports storing extra attributes for user. With this change,
extra attributes can be stored.

https://github.com/openstack/keystone/blob/c4c8d0b99a0404f4dcdb2f87c48fe15ee1526197/doc/source/api_curl_examples.rst
Add Mocha for mocking/stubbing in tests
Create a dummy provider for testing the core classes
Refactor Session tests
Add Session#request
Initial code documentation
Don't compare JSON strings

Comparing JSON strings is unpredictable since hash key ordering is
implemented differently across Ruby versions.
Create a custom Mocha expected_parameter matcher

Comparing JSON strings is unpredictable since key ordering implementation
varies between Ruby versions. This change ensures that we are comparing
between Hashes and not their string represenations.
Bump version to 0.1.0
Add Aviator::Response#to_hash
Accepts params as a hash
Add CONTRIBUTORS.txt