Support prefix and exact route at same path

This is sometimes needed when the landing page for a thing is handled by a different app to the rest of the thing.

This has involved changing the triemux to store 2 tries, one for the prefix routes, and one for the exact routes.  This also now uses the GetLongestPrefix method to do the lookup in the prefixTrie, which means that each path lookup now involves at most 2 trie lookups.

Also fixed an issue in the tests where the DummyHandlers were all equivelent, and therefore tests would still pass if the wrong handler was returned.

Note: this depends on nickstenning/trie#1