Remove EmptySampleSetException in favor of return 0

Addresses Issue #6 on Comcast/jrugged.  Instead of returning an
EmptySampleSetException if calling getQuantile or getPercentage when
there's no data for the time span, return 0, which is friendlier to
humans and machines both.
Merge pull request #13 from clinepeter/emptysample

Remove EmptySampleSetException in favor of return 0