Fail silently when trying to do a concurrent build.

When watching, concurrent builds are frequent. Throwing errors kills the
watcher.
nicer demo page
Don't use the fade transition on modals.

Immediate presentation provides a snappier experience, and the fade
transition itself is buggy under edge case scenarios.
removed log statement.
Responsive tab headers.
Added options for fine tuning cell behaviour.

One can configure the cut-off limit for long cell values.
Fixed behaviour of cell previews.

We can gain better control of the popovers by using out own subclass of
the bootstrap popover utility.
don't complicate things. Just put the tooltip in top.
Nicer cell behaviour. Some modal adjustments.