Rename .u-* to .util-*

[martymcguire]

I'm using Skeleton (it's great!) in a project that also deals with microformats2, which specifies that u-* classnames are used to indicate url properties of machine readable markup.

The result is that anything marked with, e.g. .u-full-width gets picked up as a url property. Details about the microformats2 parsing convention can be found here: http://microformats.org/wiki/microformats2#naming_conventions_for_generic_parsing

Renaming .u-* to .util-* avoids this issue while potentially making it clearer that these are utility classes that apply inline styles.

Kevin Marks has a great detailed explanation of the issue here: http://www.kevinmarks.com/u-means-style.html