MiddleClick Mavericks

Added new cask for open-source application MiddleClick for Mavericks.

__Note__: _Not entirely sure about naming conventions here. Developer has multiple binaries of this app on their homepage, each one for different recent OS X versions (Mavericks, ML, etc.). This cask specifically installs the Mavericks version, as I only have a Mavericks machine to test on, hence I've named it "middleclick-mavericks". The .app file is simply called "MiddleClick.app" though, which will most likely conflict with casks made for the other versions of MiddleClick... I've just discovered homebrew-cask last night, so if you've encountered a similar situation with other applications some suggestion as to what action to take would be appreciated_ :)