#17 Navbar improved

1. After migration to new Bootstrap version there was a bug at our navbar. It becomes collapsed for small screen resolutions what we don't wont to.For extra small and small resolutions our nav items change from text into icons so that we don't need any collapse effect. To fix it I have deactivated the collapse plugin and changed the grid-float-breakpoint to zero in the customized Bootstrap files.

2. At the new landing page, at first the navbar is invisible. Only a small green bar is visible. If this bar reaches the top of the window it will expand to the whole navbar. It is realized with the affix component of Bootstrap and some css transitions.