Add some Shell.* wrappers around bitlash doCommand

Centralizing our interactions with bitlash, and making it easier to use Shell from a sketch.
I think we can just return a StringBuffer here (by value). I'm not really fan of returning string pointers pointing to global variables (due to re-entrency problems) and I think that due to Return-Value-Optimization (IIRC) there is not even any copying of strings involved when returning a StringBuffer.