Fix #281. Add mouse-over precise time and on-click switch listener.

This fixes #218.

It introduces a new config variable, to specify how dates should be rendered.
It accepts either a string in the format of go's time package or one of the time constants of the package ([see golang.org](http://golang.org/pkg/time/#pkg-constants))

With this commit all uses of the TimeSince function in templates return a <span> with a special class and title attribute, containing the formatted date.

**Clicking on such a span will swap the title with the text**.

@Unknwon any problems with that?