Starts iOS build command and improves performance

The new implemented features are described below.

1. A new sidebar menu command called 'Build iOS'. It is only visible when you are working on OSX because it's impossible to build an iOS lib in other os. (the ios build command is not done yet)
2. To improve performance and avoid locking sublime, the build commands now run on a separate thread.
3. The 'os.system()' was replaced by 'run_os_command()' which raises an exception if the command returns an exit code greater than zero.