Added basic auth check method integration point

If you setBasicAuthCheckMethod(yourmethod) while configuring Relaxation,
it will trigger Relaxation to look for basic auth credentials and hand
them to your method. If your method returns true, everything proceeds.
If your method returns false, Relaxation will return appropriate headers
to the http client to prompt for basic authentication credentials.
Merge pull request #21 from cfchris/basic-auth-integration-point

Basic auth integration point