Adds ST3 compatibility

Includes a few fixes for the python 2/3 switch, and other breaking ST API changes:

- Proper string/byte handling
- Support ST3 `show_quick_panel`
- Split insert into helper command to handle losing `edit` context on async commands
- `print` has to be called as function

All changes should be backwards-compatible with ST2
Merge pull request #5 from diurnalist/feature/st3

Adds ST3 compatibility