Adding commonly used slugifying support to illuminate/support.

I've [previously stated](https://github.com/laravel/framework/issues/69#issuecomment-12345704) my objections to having a full-blown string class / package in Laravel 4 due to the fact that many applications do not require it and there are [many](https://github.com/meido/str), [many](https://github.com/Anahkiasen/string) different [packages](https://github.com/lembubintik/dictate) out there to cater for [different needs](https://github.com/doctrine/inflector).

However, I think `slugifying` strings is something many applications will use. Whether it be for your next simple blog post to have friendly URLs, or your next product management suite to give automatic SKUs to products (I could go on), most apps have a need to create a simple, URL friendly string.

I've done exactly this, by adding a `Slugify` class (open to suggestions on the name), with a `str_slug()` helper.

This PR relates to issue #69.