Implemented MeasurementSerializer
Implemented MetricSerializer
Added a measurements accessor to metric

If not set it defaults to an empty array
MetricSerializer can include measurements
Refactored MeasurementSerializer#to_json
Corrected spec filename
Require submodules
Added rack-test as a dependency
Require rack/test in spec_helper
Added the Server and the /status endpoint
Added .test.env to .gitignore
Implemented MetricMapper#all
MetricSerializer#to_hash returns string keys
MeasurementSerializer#to_hash returns string keys
Load .test.env in spec_helper
Added the /metrics endpoint
MetricMapper#find handles string ids
MeasurementMapper#find handles string ids
Added the /metrics/:id endpoint
Indentation