remove @shadow instances and replace with escaped values, update mixins to not use a default value at all

1. Removed borders on .btn and increased line-height 2px to compensate
2. Set buttons to use the endColor of the gradient as the background-color by default for extra contrast between button and background
3. Set a 5% darker background-color on hover of buttons just for IE7

fix other instances of mixins, swap use of box-shadow everywhere, include border-radius and transition

removing duplicate reset filter since we changed the main button to use buttonBackground mixin, per #2024

Clean up inverse button using variables, adding button group examples. Also adding inverse inline label

standardize default button gradient styles to match other buttons, reset it's IE filter to improve IE7 hover

rearrange some of the docs on buttons to show examples of all possible renderings along with code snippets and note about usage; also fix the portrait tablet media query