JoinVisitorImpl is now parameterized by two r types (left and right). first part of #228
hash cannot be defined like this
Merge branch 'master' of https://github.com/hadley/dplyr
allowing join visitor< INTSXP, REALSXP > for automatic promotion to numeric in the result data
slightly more generic code.
mv some code to join.cpp
allowing join visitor< REALSXP, INTSXP|LGLSXP > #123