added initial join visitor
added DataFrameJoinVisitors
using VisitorSetEqual as a policy to generate the equal method in DataFrameVisitors and DataFrameJoinVisitors
policy generate the hash method
policy generate less and greater
generalize DataFrameVisitorsHasher
generalize DataFrameVisitorsEqual
generalize less and greater predicate
generalize DataFrameVisitorIndexMap and DataFrameVisitorIndexSet
starting using JoinVisitor
Merge branch 'master' into cpp_join
initial version for semi_join
initial version for anti_join
benchmarking semi_join and anti_join
inner_join
left_join closes #55
uncomment benchmarking
sparing the extra copy from std::vector<int> to IntegerVector closes #54
comparing with pandas
minor cleaning