Don't overwrite the brain when loading from mongo

This updates the current mongo brain to use merge instead of overwrite for the brain loading.  We can switch the brain out for a smarter one without losing everything :smile: 

Relevant diff:
```diff
--- ../hubot-scripts/src/scripts/mongo-brain.coffee	2014-03-21 15:33:20.000000000 -0700
+++ scripts/mongo-brain.coffee	2014-04-16 11:41:07.000000000 -0700
@@ -58,7 +58,7 @@
         robot.logger.debug Util.inspect(results, false, 4)
 
         if results.length > 0
-          robot.brain.data = results[0]
+          robot.brain.mergeData results[0]
           robot.brain.emit 'loaded', results[0]
         else
           robot.logger.debug 'No results found'
```

Tagging @doomspork / @jmacadam for review