add a matrix dispatch for as.tbl_cube
Fix bug in top_n function to return specified rows

tbl %.% ... %.% top_n(n) now returns n rows instead of n-1.
Add test for top_n function

To make sure it returns n rows.
Simplify top_n test
Add top_n bug fix to news
initial code for distinct #97
slightly better impl of distinct (less hashing). #97
initial impl of union. #97
roll back as we already have union, intersect, ... #97
hybrid versions of mean, var, sd and sum handle the na.rm argument. #168
hybrid versions of min and max. #168
special handling for summarised variables in hybrid functions in summarise
special handling for summarised variables in hybrid functions in summarise
typo
initial code for handling first(.) and first(., order_by = .)
handling some more cases of first
last(.) and last(., order_by = . ). #133
testr fix
added hybrid nth using c++ nth_element. #133
percent_rank. #133