log messages in json

Resolves #357 

Output now looks like this, for example:

Apr  7 11:17:03 [ 1 ][ INFO ] {"tick":"2014-04-07 11:17:03.699989441 -0400 EDT","url":"http://citibikenyc.com/stations/json"}

Apr  7 11:23:21 [ 2 ][ INFO ] {"Count":0}