Custom app scheme

Right now scheme for `xcodebuild` and app name for `instruments` are calculated from workspace file name. I think it would be useful to provide ability to change this behaviour. For example, I'm using special target/scheme for `UIAutomation` tests with networking stubs.
 
I have added argument `:app_name` for  `rake` tasks `:compile` and `:test`(including device specific tests). This :argument is optional and can be omitted.