Add support for metrics and data_points

Adds two new models and two new collections to handle the metrics
attached to a check and the data_points available on a metric.
Bump version to 0.2.16
Add variance, min, and max to DataPoint
Make fetch more tolerant to alternate input
Add documentation for DataPoints#fetch method
Merge pull request #5 from racker/add_metrics_support

Add support for metrics and data_points