Add histogram column families
Set default TTLs for histogram rollups
RollupReadContext to hold destination granularity
Add HistogramRollupRunnable to compute histograms
Add Histogram computations to rollup pipeline
Add integration test cases
Turn off histogram computations by default
Add changelog to CHANGES
Throw RuntimeException on invalid Granualrity
Truncate histogram column families after tests end
Fix merge conflict issues
Merge pull request #164 from lakshmi-kannan/bf_integrate_histograms_pipeline

RFR: Integrate histograms into rollup pipeline
Refactor BasicRollup output serializer and fix a bug in tests
Fix bug in HistogramRollup when number of bins computed could be -ve
Expose get methods for histograms in AstyanaxReader and RollupHandler
Add histogram JSON output serializer and test
Expose HTTP endpoint and add HTTP handler for histograms
Add tests for hisotgrams HTTP endpoint
Add changelog to CHANGES
Switch to codahale metrics instead of yammer