added tbl_cpp and grouped_cpp
export tbl_cpp and grouped_cpp methods from namespace
added all_equal_ for comparing multiple df at the same time
added group_by.tbl_cpp method
convert more demo to using tbl_cpp
equal_ treats NA as equal. closes #73
more uses of tbl_cpp
internal impl of group_size. closes #65
internal build_index. closes #63
using 0-based indices
structure the result of summarise as tbl_cpp objects
filter correctly produces tbl_cpp classes. Part of #69
mutate sets attributes and classes. Part of #69
trimming unused
using set_rownames
internal handling of lazyness in GroupDataFrame ctor. closes #64
simplify build_index_cpp interface. get back at the same intf as build_index
added train_push_back to avoid code duplication
more handling of output classes
arrange handling of class. closes #69