remove modal-open class
hopefully resolve z-index headaches

fixes #5102, #5166, #5194, #5205: add overflow: visible; to .nav-collapse.collapse so that dropdowns overflow correctly in the navbar in IE7-8

add new vars for common border-radius, upping border-radius of inputs to 4px; add border-radius changes to -large, -small, -mini for buttons and paginations

add new vars for common font-sizes and paddings with large, small, and mini components; resize pagination and buttons to better match each other

fixes #5177: remove color on .btn-large .caret, but keep sizing change; also start to add CSS tests for buttons

Add CONTRIBUTING.md

Signed-off-by: Chris Aniszczyk <zx@twitter.com>