Implement Comparable inteface on Locator

Implemented the Comparable inteface on Locator to fix a
ClassCastException error. Here's the full error:

    java.lang.ClassCastException: com.rackspacecloud.blueflood.types.Locator cannot be cast to java.lang.Comparable

This occurred when using the contains method on some Set class to see if
the Locator was already present in the container.

Fixes batched reads in my dev environment. :)