Fix text not displaying precision properly

Was using parseInt on the result of roundStep for ceil, floor etc.,
which was ruining the formatting.
Split slider into slider (for single values) and slider-range (for ranges)

Unfortunately both have slightly different HTML, which used to be handled
by compiling different templates, but that doesn't work inside transcluded
scopes. brianmcd committed a workaround, which was to use a link function
and no templates, but this was by his admission a hack which only worked
with range sliders. This commit duplicates the directive into 'slider', a
single value slider, and 'slider-range', a range slider.

The code for each is surprisingly different, so there's not a huge amount
of duplication, though I could definitely extract some common
functionality out.
Fix selection handle being slightly offset from centre
Add onChange hook, called every time the slider is moved
Delete slider-template.jade. It doesn't seem to be used