use button elements instead of anchor elements and add tooltips to zoomcontrol

TL;DR: if it is a button it should be a button.

Using the correct semantics serves accessiblity and adding a tooltip (which is screen reader accessible) makes the button easier to understand for everyone. Also proper focus handling is important for keyboard users since the have no `:hover` event

the tooltip can be easily disabled by removing the `:focus` and `:hover` pseudo classes or  using some simple css to override those.

more background in: http://www.geodienstencentrum.nl/blog/accessibility/webmapping/2014-02-14/enhancing-openlayers-controls.html