Expose API to get longest prefix match / "closest" keys

[GoogleCodeExporter]

Please add support for querying the longest prefix match (this is not currently 
exposed in the public API). A variation of this which only matches if the key 
is in fact truly a prefix would be useful, otherwise the caller would have to 
run an additional prefix.startsWith(key). For example, if the tree only 
contains "foo", it's the longest prefix match for anything. But in the use-case 
I have, I would only want to match "foo*".

Original issue reported on code.google.com by phraktle on 19 Nov 2012 at 10:33