On windows, insert path with forward slashes instead of backslashes

Example:
Before changes a path would get inserted in the following way:
"collections\links", which is problematic because now the backslash
escapes the next character instead of delimiting a directory.  After the
changes it will get inserted like this: "collections/links".
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 #3 from IsaacAndela/master

On windows, insert path with forward slashes instead of backslashes
Merge pull request #5 from diurnalist/feature/st3

Adds ST3 compatibility
Removing ST2 backwards compatibility, will be maintained in separate branch