MiddleClick

Added new cask for open-source application MiddleClick for Mavericks.
Modified previously made cask for MiddleClick.

Changed cask and class names to middleclick.rb and Middleclick,
respectively. Added caveat stating OS requirements.
Modified MiddleClick cask version scheme.

Changed version to follow 'latest' scheme and removed checksum.
Merge pull request #2079 from poltak/middleclick

Modified previously submitted Middleclick cask