Issue 81 scheme fix

Fixes an issue with passing the scheme (first discussed here: https://github.com/nomad/shenzhen/issues/81).

The '-sdk' flag was overriding the '-s' flag for scheme. It seems as though the options parser was only taking the first letter of the command option. I have changed it so that there is no '-sdk' flag and you should use the '--sdk' flag for specifying the sdk.

I've also changed the configuration validation. If the user is specifying a project, the gem validates the configuration (as it did). If the user is specifying a workspace, this validation is skipped. The 'xcodebuild -list' command does not list configurations for workspaces. The 'xcodebuild' command will fail, however, if the specified configuration does not exist. For workspaces, the default configuration is Release (which should be fine). You can still specify Debug, however.