Automate compile and minify less files oc-admin

Now each time we want to compile and minify the less files of the oc-admin we do it as we want... a repetitive task.

I've created a task using [Grunt](http://gruntjs.com/) that compiles and minifies the less files in the right css file. So, each time we want to compile and minify the less files we only have to use:

```
$> grunt compile
```

Before doing this we have to make sure we have the following requirements: [nodejs](http://nodejs.org/) and install the dependencies `npm install`.

The reason is because Grunt is a well-known and maintained library used by projects su as jQuery, Modernizr, etc.

What do you think? The idea is getting the manual task done here.