Merge tag '1.6.0' into develop

v1.6.0
- new: RateLib added
New: add AbsoluteLimiter::getUnitsSeen()
Merge branch 'develop' of git://github.com/datasift/ms-lib-stone into develop
Fix: added AbsoluteLimiter::getUnitsSeen()
Merge remote-tracking branch 'origin/develop' into develop
Version bump
Merge branch 'release/1.6.1'