Now presents correct initial state to sample function when a slot is added during update
Now manually calculates mean (average) value for a histogram.
Added functionality for externally provided mean (average) value to get_statistics
Fixed typo level bug, which may or may not affect Ulf's tests.
Merge branch 'uw-exometer-report' of github.com:Feuerlabs/exometer into uw-exometer-report