Implement functionality to merge multiple tries

[cowtowncoder]

Given iterator functionality (see http://github.com/ning/tr13/issues#issue/2), it should be easy to implement merge functionality, where 2 (or perhaps more) input tries are merged into a result trie.
This assumes that tries are ordered the same way (meaning usually they are not reordered).

Functionality would be most useful for incremental building of tries, for example in (near) realtime cases where mutable HashMap (or such) is used for gathering new entries, which are regularly merged with immutable tries instance, to produce new immutable version.