"marathon start" now checks if an app id is already running, handles scaling a...

...nd resource changes.

I wanted a single command that, given an app definition, would check if its already been started, start it if not, but also handle scaling or changes to memory or cpu requirements if the specified options don't match what's running.  I decided to get there by overloading the meaning of the "start" option here.  Not sure if it makes sense upstream but just in case :)