Allow use of lessphp

Regarding... https://github.com/SpoonX/SxBootstrap/issues/39

I've made a change that should allow the user to set a config variable to allow them to use lessphp instead of node.js. I've also modified the readme (as a suggestion) to note the new installation method. Essentially, they have to require lessphp themselves via composer, and add 'use_lessphp' => true, to their 'twiter_bootstrap' config array.