add support to less files used in plugins

Allow to route the Helper to a .less file located in the webroot of a plugin. Usage: echo $this->Less->css('yourfile', array('plugin' => 'PluginFolderName'));
Update usage section

Add option to add a less file located in a plugin
Merge pull request #10 from ybenassuli/master

Add support to files located in a plugin webroot
Merge branch 'master' of https://github.com/Hyra/less
Added composer.json file
Remove gitmodules