Remove dependency on Google Collections

While it's nice to be able to simply reuse these "standard" collections, doing so results in an extra dependency on all our modules and our users. When using Maven this is not really a big deal, but the extra JAR is annoying whenever the dependencies have to be explicitly managed.

We shouldn't be using too many of these classes, and are probably mostly using the MultiMap. We'll likely have to recreate something similar in our code (in o.m.common.collections).