Update documentation for CasperJS helper methods.
Rename CasperJS variable file.
Update documentation for variables.
Update documentation for Drush and PhantomCSS scripts.
Explicitly compare generated diffs.

PhantomCSS was sometimes skipping running visual tests. Explicitly
giving an array of tests seems to fix this issue.
Merge pull request #3108 from DFurnes/test-docs-and-phantomcss-bug

Test docs and phantomcss bug