Stop test server with ctrl-C when run standalone
Can run the server on a port other than 8000

Allows to have the server on a user defined port when run from the
command line. Also, command line default is now 8080, to avoid conflicts
with tests (that run the server on port 8000).
Rename HttpStub -> TestServer (it's not a stub)
Rename test server file
Merge branch 'master' into stop-test-server
Merge pull request #21 from alphagov/stop-test-server

Test server improvements