Importing dplyrRcpp's code

This is the initial import we discussed in emails. All is isolated from the rest of dplyr's code at the moment, i.e. your code does not call this code. 

This contains: 
- inst/include: headers for things from dplyrRcpp
- src/*.cpp: implementations
- R/internals.r: functions ending with `_` (`mutate_`, ...) that call the internal code.
- demo/bench-*.R: the code that was in r_examples in dplyrRcpp