Add select_ to support later dplyr

There are two problems when supporting R version 3.1:
* The string of conversion seems to act differently, so I replaced it with no conversion (seems to work when `data` is a dataframe)
    dat <- tbl_df(as.data.frame(lapply(1:ncol(data), function(x) type.convert(as.character(data[,x])))))
* dplyr now contains and uses a `select_` function, and I put one in that appears to work ok (though there could still be issues with it)

Still to resolve
* How to do the conversion correctly for R 3.1
* How to export `select_.treedata` for R 3.0 when select_ is not defined. I attempted this in an `else` block, but someone would need to confirm on R 3.0 since I upgraded my system and now don't really want to go back :) - Or maybe we just don't support R 3.0 anymore and force people to upgrade.