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

This is an implementation that behaves similar to 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 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!