Convert Rx Observables to Ratpack Promises. (Issue 468)

Supports testing observables by converting them to promises and using
the existing ExecHarness methods. (Issue 384)
Turn off some styling that doesn't work with latest version of Sass.
Merge pull request #474 from johnrengelman/rx-test

Convert Rx Observables to Ratpack Promises. (Issue 468)
Some polishing of support for converting Observables into Promises.

Closes #468.
Remove unused method.
Make the HikariModule a ConfigurableModule.

Supersedes #456.
Merge remote-tracking branch 'upstream/master'