[COLLECTIONS-800] Adding partitionBalanced(List,int) method

[ClaudioConsolmagno]

Returns consecutive sublists of a list, partitioned in a way to balance entries across all sublists. See COLLECTIONS-800 for more details.

Implementation Notes:

• Given that the Partition class is a private static class not used by anything else I think it's ok to just use isBalanced as a boolean instead of, say, an enum or having a separate brand new private class for this new method.
• Added several comments to clarify the logic, let me know if you think that's too much.
• I didn't know how far I could go when refactoring existing tests so I only half changed them for partition() method.
• Similarly, existing tests all around seem fairly simple but I thought I should have good coverage and test different scenarios for partitionBalanced() hence added the whole forEach test cases (I'm a fan of Spock and data driven testing).

[coveralls]

Coverage increased (+0.002%) to 90.139% when pulling 4557a45 on ClaudioConsolmagno:COLLECTIONS-800-partition-balanced into 7ab4e4f on apache:master.

[LarsBodewig]

Newbie myself, but I wonder if that could be written more simply:

evenlyDistributedSize = ...
if (index < threshold) { // lists before threshold should contain +1
    start = index * (evenlyDistributedSize + 1);
    end = start + (evenlyDistributedSize + 1);
} else {
    start = index * evenlyDistributedSize + threshold; // offset other starting positions by threshold
    end = start + evenlyDistributedSize;
}

...or similar (Note: this is untested). The if condition and subtractions look quite complicated in my opinion.

[LarsBodewig]

I see why you would use the example from ListUtils.partition, however this could be confusing IMO if you start comparing what method to use and see no difference so far. I would opt for a more clear example here.