reduce the number of IE specific overrides. adjust margin for radio and checkbox.
Add visibly hidden button text for screen reader users.

BS3 commits: 31dfc3218e4aafca3a3963a632a22ee018362c00
fix swapped angle brackets in docs; fixes part of 10456

BS3 commits: 0b0087f3a744e5e1e86a87cfa5c929b34e3ddd86
add <code> around ".less" in Getting Started docs

BS3 commits: 121790c1062d8057b5bddedb3960ec7ceedb498d