Install Mocha, loading test environment in test/helper.rb .

Also remove unused `test/ryespy_test.rb`.
Implement App#start,#stop , moving executable main loop.

Add `App#initialize`,`#start`,`#stop` for application run control, with
`#setup`,`#cleanup`,`#refresh_loop` hooks. Move main loop from executable
to `App#refresh_loop`, setting up listener outside of loop. Trap
`SIGINT`, calling `App#stop` for friendly shutdown. Wrap executable
within rescue for friendly reporting of large errors, using `--verbose`
to toggle stacktrace.

Note that much of this code is based on the run control implementation in
[sidekiq-spy](https://github.com/tiredpixel/sidekiq-spy) (MIT License).