Add support for username and password authentication

[sixfeetover]

Fixes #200, and maintains support for auth keys. Also added tests for parsing URIs with and without usernames.

[nviennot]

Hey Jeff, thanks for the PR :)

I've modified your commit with the following:

• Simplified the logic with password vs auth_key in the implementation (the driver is backward compatible with the auth_key) (and no need for ifs)
• Remove the tests you wrote (they don't really do much: e.g. if the rethinkdb driver changes its implementation we will still go green -- and that's really what we need to test, but we won't because it involves reconfiguring the DB)
• Added the environment variables RETHINKDB_USER and RETHINKDB_PASSWORD and made some tests for that

The commit is https://github.com/nviennot/nobrainer/commit/0b3ca76449ae264d59df08af8c933ee248ac083d

Let me know if you approve as the commit is in your name :)

[sixfeetover]

Looks good to me! I wasn't sure how important it was to preserve auth key, so I took the conservative approach. Thanks for the quick review. :shipit:

[nviennot]

I looked at https://github.com/rethinkdb/rethinkdb/blob/next/drivers/ruby/lib/net.rb#L485 and though it wasn't :)