Add support for {{ $foo or 'bar' }} in Laravel 4.0.*.

Hi,

Without breaking any backwards compatibility, I thought it would be extremely nifty and would certainly reduce a lot of code in people's projects to have the `{{ $foo or 'bar' }}` (variable fallback / default) functionality back ported to Laravel 4.0.

~~I also added tests which could be merged back into 4.1 development if you'd like.~~ **Scrap that, they are. I'll put those specific tests into my PR and update it.**