Implement backend selection double slashes tests

Using gomega ghttp recorder.
Prevent HTTP request helpers setting a User-Agent.

The Go http library sets one by default, so we need to prevent this.
Port proxy header handling tests
Port remaining proxy function tests to Go
Initial performance test.

Basic test comparing router with direct requests.
Port remaining performance tests to Go
Port route loading tests to Go
Port reload API specs to Go
Port error handling tests to Go
Remove remaining bits of ruby test suite.

There are no longer any tests using rspec, so nothing depends on ruby
any more.
Update README to reflect changes to testing.

This also includes changes to reflect the recent switch to gom in #74