Merge pull request #165 from trestletech/master

Export validateCSSUnit function
Update reactives.R

Add newline (guessing that's how it's supposed to be)
Document observer methods
Remove annoying title="foo" tooltip on all tabset tabs
Merge pull request #170 from hadley/master

Fix typo
Bump version to 0.6.0.99 for development
Add option for not using Cairo
Merge pull request #172 from wch/cairo-option

Add option for not using Cairo