Add OAuth.deleteUserData method

Removes the user's oAuthId-to-uid mapping from Redis upon user deletion.
This fixes an issue where OAuth services could not link to a user if a
user account was deleted and then re-created.
Fix OAuth.deleteUserData to use filter hook

The action:user.create hook in NodeBB was recently changed to
filter.user:create, which allows the OAuth.deleteUserData method to remove
OAuth data for the user more directly.
closed #4
Merge branch 'master' of github.com:Fastinyoh/nodebb-plugin-sso-oauth into Fastinyoh-master

Conflicts:
	plugin.json
static:app.load
0.1.3
simplifying the plugin a bit for new users
bugfixes
more bugs
beter intro message
0.2.0
updated readme
Merge pull request #6 from julianlam/upstream

updated readme
more keywords and removed unused template
0.2.1
bringing codebase inline with new upstream changes