More ST3 support (when output goes to panel)

More ST3 support (used the example of Git plugin to figure out how to do this properly):
* Text escaping changed in python3 (had to alter _make_text_safeish, subprocess.check_output was not sufficient).
* Output to panel works differently in ST3 (begin_edit, end_edit are not available, as per http://www.sublimetext.com/docs/3/porting_guide.html)

The code still works in ST2, at least as much as I tested it.