Internal implementation of select

[hadley]

To avoid making any copies of the columns (a la http://stackoverflow.com/questions/20886748)

[hadley]

Before implementing this, need to implement #158 and #192

[romainfrancois]

An easy enough way to handle this is to compute the names of the variables we want to keep on the R side and then pass the original data frame and these names down to some C++ function that will use shallow copying of the columns.

But we should I think keep the logic in R. I mean we could do #158 internally if we want, but that's not going to be very worth it.