Equals for cluster subclasses

[hypest]

This is a re-submission of #169 since I no longer have commit access to that one.

For reference, here's the original PR introduction:

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!

Let me know if this causes any problems. Thanks!

[hypest]

It seems that the build is failing in Travis but not for reasons related to the changes themselves:

Execution failed for task ':library:connectedDebugAndroidTest'.
com.android.builder.testing.api.TestException: com.android.builder.testing.api.DeviceException: com.android.ddmlib.ShellCommandUnresponsiveException

I think re-triggering Travis might give this a chance to succeed. @markmcd would you mind restarting the Travis build on this one? I can try to do it from my side by closing and re-opening this PR but I'll wait for your input first. Thanks!

[markmcd]

Yes indeed, apologies. Builds are flaky at the moment.

Looks good - thanks again.