Added new helper function to auth, user_exists

Simple function, have replaced any user existance checks in the code to this new function. This will be useful once alerts are in place
Added global alert id (aid) counter to keys
Moved alert stubs to zset from list.

Sorted sets is what we need for alerts. This will allow us to only notify on new messages based on the time the user last visited /alerts and the timestamp used as the score in the zset
Moved to flask_* names for extension imports.

The flask.ext.* is being deprecated. I have moved all imports of Flask extensions to use the new format. This helps with pylint'ing as pylint can't deal with the flask.ext.* , magic imports.