Equals for Cluster subclasses

[hypest]

Adds implementations of "equals()" and "hashCode()" in StaticCluster and QuadItem classes. Benefit is the ability to reliably compare (and identify) instances of those classes.

As a bonus (and an implicit way to test QuadItem::hashCode() without exposing the inner class) this PR also includes an implementation of NonHierarchicalDistanceBasedAlgorithm::removeItem(). By the way, this would effectively fix issue #133.

PR commits include relevant tests. Let me know if more testing should be added.

Also, the added test files are new and thus copyrighted after myself, but still under the Apache v2 license. Let me know if this needs to be revised somehow.

Cheers!

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[hypest]

I signed it!

[googlebot]

CLAs look good, thanks!

[markmcd]

Code looks good - thanks for taking the time to write tests, too.

With the copyright headers, can you please change the (c) line to Copyright 2015 Google Inc.? You are more than welcome to add yourself to the CONTRIBUTORS.md file (but you don't have to if you don't want to).

[hypest]

Thanks for reviewing @markmcd!

More than happy to make the changes! Thing is, I no longer have commit access to this fork so I can't commit the changes. For that, I opened a new PR (#245).

Let me know if there's anything more I should do with the current PR.

Thanks in advance!

[markmcd]

Dropping in favour of #245.