add geomean(interval) + reduce deps

[aplavin]

Statistics.mean(interval) already works, so it's only natural to add StatsBase.geomean(interval) – that's what I do here.

Also reduce deps on stdlibs:

• Dates are just not needed, I simply removed using Dates
• Made Random a weakdep on 1.9+

So, now IntervalSets is completely dependency-free (on 1.9+), even regarding stdlibs!

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (6545e5d) 99.10% compared to head (f12cfd7) 99.11%. Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #171 +/- ## ======================================= Coverage 99.10% 99.11% ======================================= Files 4 6 +2 Lines 224 225 +1 ======================================= + Hits 222 223 +1 Misses 2 2 ```

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