cache array length for loops

[Erik Blankinship]

This is a minor minor minor enhancement

Many of your loops could benefit from a simple optimization of caching the length value of the array you are looping on, for example:
For example:
https://github.com/Box9/jss/blob/master/jss.js#L102
into
for (i = 0, iMax=rules.length; i < iMax; i++) {