Append with method to_bson(encoded) for serialization

Significant performance improvement gained for encode by supplying 'encoded' string buffer to method to_bson.  Performance measurement results can be viewed at the following links.  Durran, please review - I thought that a pull request would be a good idea for this rather than just pushing to master.

http://gjmurakami-10gen.github.io/working-notes/ruby_bson_performance/append/test_bson_all_append_denizen.html

http://gjmurakami-10gen.github.io/working-notes/ruby_bson_performance/append/index.html