add .navbar-static-bottom

1) Add new class .navbar-static-bottom. Now create a static footer so easy like a static top navbar! Example of static footer is in "example" folder.

2) Typo fixed, changed .contant to .content, refresh .less files (grid, navbar, normilize)

3) Make .navbar-fixed-top/bottom like a .navbar-static-top/bottom for screen less 320px in height and landscape orientation.