Added new calculated stat type

Added stat type to listen for changes to other stats and invoke a calculation function on those stats to produce a result
Added Stack Driver backend for flushing of Stats to Stack Driver API
Various fixes to StackDriver backend

* Convert the unix timestamp passed in to an integer, to comply with StackDriver API
* Set the stat value to output toString() before posting to StackDriver
* Fixed context to StackDriver object members to use self=this
Fixed initialValue and resetValue for all stats to allow for 0s
Updated test cases
Updated Changelog and Readme, bumped package.json to 0.1.0, and updated keywords