Add cl-trie

cl-trie is a library that implements a trie(known as prefix trie).

Code and documentation can be found at https://github.com/MatthewRock/cl-trie

It contains both cl-trie(the library) and cl-trie-examples(examples). Should I send separate PR for examples?

The library is tested with each commit on Travis CI and currently builds on ABCL, SBCL, Allegro, CMU, CLL and ECL.
