Removed empty method definition
MeasurementMapper exposes COLL_NAME
MetricMapper exposes COLL_NAME
Implemented MeasurementMapper#create
Refactored MetricMapper spec
Implemented Measurement#==
Implemented MeasurementMapper#find
Implemented MeasurementMapper#load_metric

Also added the optional option :load_metric to MeasurementMapper#find.
If set to true this will cause the metric that the measurement belongs
to automatically be loaded.
Implemented MeasurementMapper#count