added search button

In some circumstances (browser, website) using enter to trigger the search leads to
side effects. So having a button to trigger the search is easier.

The searchbutton can be activated with
  searchbutton : true

The text of the searchbutton can be set with
  searchtext : "Search"
Merge branch 'master' of https://github.com/cornelinux/Flexigrid

Conflicts:
	js/flexigrid.js