Support JRuby

[mrkn]

For supporting JRuby, we first need to let enumerable-statistics support JRuby.

[mrkn]

red-data-tools/enumerable-statistics#20

[schneems]

I think this could be somewhat related to #31. If we made a pure ruby version of this library then it would support Jruby out of the box. Then the only two other places (i think) in here that are actually using enumerable-statistics are Array#histogram and Array#percentile.

I've already taken the liberty of writing a pure ruby version of Array#histogram here github.com/zombocom/mini_histogram. It matches the interface provided by enumerable-statistics on purpose. Array#percentile would be pretty easy to code up.