Improve plugin loading logic to fix loading on Win with ST3.

In ST3, the script path is of the included module, in ST2 (Python2?) the path
is relative to the includee. Make the loading logic more flexible by looking
up 3 directories up from the current one, looking for the plugin.
Fix typo.
Merge pull request #218 from rchl/master

Fix for plugin loading on ST3 + Windows