Feature absolute urls in tests, override command line URL

[Kesmy]

Minor addition of a command-line argument (--absolute-urls) which
discards the base URL provided on the command line and treats
the "url" in test cases as an Absolute/Full URL.

Default test:

- test:
    - group: "Smoketest"
    - name: "API Root"
    - url: "/users/octocat"
    - expected_status: [200]

Test using --absolute-urls:

- test:
    - group: "Smoketest"
    - name: "API Root"
    - url: "http://api.github.com/users/octocat"
    - expected_status: [200]

[Kesmy]

This feature is something I added locally for my own use.

I'm working with an API that returns full URLs in responses, and I wanted to use them directly, without any additional parsing, in future tests.

resttest.py still requires a URL parameter, this switch simply changes it to an empty string before building/running tests.

[svanoort]

Looking good, and cleans up a few dangling style points!