Run by test id and abort running tests.

Adds the ability to start tests by listing ids with the --tests option as specified by a blank POST request to https://app.rainforestqa.com/api/1/runs.json:
    {
      "error": "You must specify the tags or tests parameter"
    }

Also adds the ability to abort runs by run id. 

Note: this merge and the list merge (for listing tests, sites, and runs) require special merging as lines 14 and 15 of lib/rainforest/cli/options.rb conflict.