Normalize decimals in IE keyword args

This PR normalizes decimal arguments in IE keyword args.

I originally created https://github.com/sass/libsass/pull/663 and closed it thinking the implementation needed work. Now a little older, a little wiser, and having consulted the Ruby sass source I'm happy with the original implementation. 

Fixes #623. Specs added https://github.com/sass/sass-spec/pull/128, https://github.com/sass/sass-spec/pull/153.