Commandline tool

Took a first stab at the command line tool described in #116

It takes the path to the _packages.json_ as first argument and a package name as the second. 

If the second argument is omitted, it will build, install and immediately remove each plugin, this can be used to check which plugins don't build at all. For example, the output which the current _packages.json_ looks like this:

    The following packages could not be installed: (
    "BTGisterPost-Xcode5",
    CodePilot,
    "XCFixin_HideDistractions",
    "XCode4_beginning_of_line",
    XVim
    )

Thoughts?