Added Setting and Syncing of Metric Trackers

I have added some functionality to py-analytics. I wanted to be able to set a certain value for a metric on a given day. This is valuable if you wanted to decrement a metric or set a metric to 0 for a given day without deleting all the py-analytics keys. Along with that capability, it made sense to create a syncing method that would update the counters for the weekly and month values, so all the data was coherent and correct.