Re-worked request interception to configure itself as the default system proxy.
When the server starts, the system proxy registers itself, and unmodified calls to any Url can be hooked and intercepted.
Currently only tested against the existing WebRequest test cases - need to verify against other libraries. Anything that respects the system proxy configuration should work unfettered.

Original "direct" use-case still works correctly.
Updated documentation to match new behaviour.
Merge pull request #3 from davidwhitney/master

Re-worked request interception to configure itself as the default system proxy