Adds two helper methods for generating HTML links based on a condition.

This is an implementation of the Ruby URL helpers documented at:

* __link_to_if__ http://apidock.com/rails/ActionView/Helpers/UrlHelper/link_to_if
* __link_to_unless__ http://apidock.com/rails/ActionView/Helpers/UrlHelper/link_to_unless

If I need to submit tests along with this, please let me know.  I noticed the [link_to](https://github.com/laravel/framework/blob/master/src/Illuminate/Support/helpers.php#L637) helper did not have tests and I assume that is because it requires the framework to be booted.  I am happy to attempt the process of writing tests for link_to* helpers!

Thanks!