Table caption for html, latex and pandoc

CF. https://github.com/yihui/knitr/issues/657
Merge pull request #687 from larmarange/table_captions

Table caption for html, latex and pandoc
Revert "there is no install-no-vignettes"

This reverts commit 4411bbb13c7ca29dc91effd2d6149b208d9dc8bf.

Conflicts:
	Makefile

- Resolved
don't use \midrule between data rows in booktabs mode
fix kable_latex output in booktabs mode and with omitted header
two new parameters to kable_latex: midrule and linesep

- By default, in booktabs mode, an \addlinespace is added every fifth line
- The last two lines are treated especially: In a table with 6, 11, 16, ... lines, no space is added before the last line
- Corner cases with 0 and 1 rows are tested by the examples
- Remove two bogus tab characters