NodesController index performance enhancement

Took a look at the nodes/index action and saw massive amount of queries, mostly for trying to load non existent association objects. 

* By using eager_load instead of includes, AR is forced to use
  joins as much as possible, cutting down on # of queries.
* Poi has a scope method to do the eager_load chains. (Name is questionable. Open for suggestions.)
* A order clause, mainly only to make old and new finder methods behave consistently. In my case, also evens out the distribution of markers on the map.

Performance tests suggest that it's probably possible to shave down overall render times by 2/3rd with this.

Additionally, I added a few things to .gitignore and deconfigured
librato to cut down on librato-rails warning messages.