Add tree.size() methods

[GoogleCodeExporter]

Currently..

    Iterables.count(myTree.getValuesForClosestKeys(""))

..can be used to count the number of keys/values in the radix tree.

This ticket is to add a size() method to the trees, to simplify this, and also 
it may be more efficient than to calculate size as above.

Note that calculating the size of a radix tree is an expensive operation having 
O(n) time complexity. However the method may be useful for debugging purposes.

Original issue reported on code.google.com by [email protected] on 3 Dec 2013 at 10:45