Initial stab at String

This is a fun idea. Took a first stab at a complete-as-possible implementation of ruby's String class. Made some go-specific changes where necessary, but tried to keep the method arguments and their cardinality consistent. Also set up some empty TBD files for implementing all the Rails extensions (inflectors, etc.). Tons of work to do still, but you can get an idea of where this is going with what I've implemented so far.