Create longstrings.rnw

This is an example of long character strings being shortened in the end document
Merge pull request #27 from eborgnine/patch-1

Create longstrings.rnw
rename the longstring example, and tweaked it
update cache
don't use \midrule between data rows in booktabs mode

cf. yihui/knitr@90effdfaf6adadc73