Allowing overrides for helper functions.

I've run into an issue which was reiterated in #527 by @billmn.

There come times where it is useful to override the helper functions in Laravel. Two scenarios I can think of off the top of my head:

1. `asset()` - I may wish to attach my own custom asset handling here. As an example, @cartalyst have an awesome Themes package which uses Assetic to lazily compile static assets, but they can't use the `asset()` function because it's reserved.
2. `value()` - I may wish to allow value to accept parameters rather than just returning the result of a Closure. This is a bit more far-fetched but the principle still applies; somebody may need this.

We're not breaking **any** BC or causing \*any\* overhead either.

Enjoy.

Signed-off-by: Ben Corlett <bencorlett@me.com>