add pod spec

Now we can put in our podfile somethings like that to fetch directly master :
`pod 'PonyDebugger', :git => 'https://github.com/square/PonyDebugger.git'`

BTW, Don't forget to change the url from https://github.com/hefgi/PonyDebugger.git to https://github.com/square/PonyDebugger.git