Add dumb Open Dart Editor command

Adds a command to open the Dart Editor from ST. Ideally it should
open the current file in the editor, and possibly at the same
rowcol position, but I don't know if this is possible right now
with the Dart Editor.

Also adds support code:

* Adds convenience function to build paths on Windows
* Adds tests for said function
* Adds OpenDartEditor plugin
* Adds unrelated docstring and __init__ method
* Updates .sublime-commands file
* Adds SDK class that wraps the Dart sdk directory to provide
  convenient access to paths, tools, etc. (Basic implementation.)