Add yard to the dev dependencies

Redcarpet is the preferred yard markdown parser.
Add .yardopts to control yard
Adds instructions for using yard
WIP Adds a copy of iOS env var docs

This is how we doc the enviornment variables.
See the .yardopts.
Add yard task to Rakefile
Yard uses doc for files; git ignore it
WIP Pairing with Tobias on env docs
WIP Removed iOS content, added @Tobias tags
Improved the env. var. docs with Tobias
Merge pull request #473 from calabash/feature/standup-yard-docs

Stand up yard docs