Joins don't need to use same vars on both sides any more.

Closes #753
Implement `as_data_frame`. Closes #749
Update readme
Convert readme to Rmd

And fix lots of problems with the code
Correct command to install from travis
eliminated weird formatting from get_single_class
Add missing data.table namespace in join methods

Closes #786
Print col names even if no rows

Closes #652
Make print a well-behave pipe
Implement add_rownames()

Closes #639
Add data frame method for type_sum
Overall dt backend.

Closes #579
Merge branch 'master' of https://github.com/hadley/dplyr
fix right_join(), add tests

correct the right frame row index to be used,
if there's no matching rows in the left frame