Extract credentials passed in Url

[russcam]

This commit extracts the credentials passed in a Url and uses them to pass Basic Credentials when constructing the single node Transport, as well as constructing a client for elasticsearch integration tests.

Leaving the credentials in the Url and also configuring credentials through .auth() fn, results in reqwest sending two Authorization headers.

[russcam]

I think it might be good to expose a function somewhere that accepts a Url and returns a (Url, Option<Credentials>), as this operation may need to be performed for all Uris passed to construct a multi node connection pool when #139 lands.