Added code coverage with coveralls.io

- added coverage badge to README
- used SimpleCov init from the cocoapods-downloader spec_helper.rb
- use ruby 2.0.0-p353, travis appears to have dropped p247 binary support (??)
- must use old style hashrocket so this will work on all supported rubies
- must install bundler on system version of ruby
- simplify the Rakefile bootstrap to not worry about travis
- organize travis.yml based on some other CocoaPods gems
- .gitignore /coverage
- explicitly filter out /travis_bundle_dir from coverage