added log.py logging plugin.

[SpaceTrucker2196]

This plugin was created with the intention of logging sensor data to
splunk. File is in json format with unit names omitted.

Sample:
{"time":"2014-01-05 10:21:59.222526","Temperature":"22.5","Pressure":"977.52","Relative_Humidity":"26.3999996185","Light_Level":"2884.13098237","Smoke_Level":"31585.7142857","Nitrogen_Dioxide":"19312.3209169","Carbon_Monoxide":"675000.0",}

[argysamo]

Hey @jkunzelman. I like the idea of logging observations to a file. However, I would like it even more, if you considered the case of .log growing very big (i.e. rendering it inaccessible). I would check the file size, and if it exceeded a certain threshold I would zip it and write observations to a new one.
Thanks for the great contribution.