Moves IE styles to their own override

Moves IE styles to their own stylesheet that appears last in the
sequence and is used to override the base styles with IE-specific
quirks. This allows:
- Removal of :after and :before selectors from the main stylesheet.
- Easier removal of legacy IE support, should that be removed in the
future.
Updates modernizr custom build

Updates modernizr custom build to include html5-shiv and include all
core features used on the site. Custom build URL:
http://modernizr.com/download/#-fontface-backgroundsize-borderradius-box
shadow-multiplebgs-opacity-rgba-textshadow-generatedcontent-cssgradients
-input-inputtypes-geolocation-printshiv-cssclasses-addtest-teststyles-te
stprop-testallprops-prefixes-domprefixes-load

Performs minor code cleanup (trailing return, remove unused comment,
remove unused script).