Add the "length" function to lessphp

[jdemangeon]

Add the famous "length" lib function to lessphp (http://lesscss.org/functions/#list-functions).

Required to use loops like this example: https://github.com/seven-phases-max/less.curious/blob/master/src/for.less, http://webdesign.tutsplus.com/tutorials/understanding-the-less-loop--cms-23827, .. or other stuff.

[Krinkle]

Seems fine from a quick glance, but should be covered by a unit test to confirm standard compliance with upstream less.js.