support choice of time units for metered rates and timer snapshots

- expand keyword-to-timeunit helper function to cover remaining case of the TimeUnit enum
- extend the rates function for metered metrics to support time units other than seconds
- extend the snapshot function for timers to support time units other than nanoseconds
Merge pull request #1 from dwhjames/topic/units

support choice of time units for metered rates and timer snapshots
Bump to 0.1.6-SNAPSHOT
Formatting and comments fixes.
Initial implementation of Ring middleware.
Update README.md

Fixing Travis-CI build indicator
Bumping to metrics 3.1.0; updated histogram test as the underlying implementation has changed.
Bumping to Clojure 1.6
Bumping to 0.17-SNAPSHOT and updating Travis config
Releasing 0.1.7
Bump Marginalia to 0.8.0 release for great justice
"Updating gh-pages documentation"
Updating docs for 0.1.4
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