Added some documentation to alert_box

Added documentation to `AlertBoxHelper` explaining that passing HTML should be accomplished by using a block rather that an argument. If a `link_to` is passed as a regular argument it wont add `alert-link` class to the link element.
@buren Currently, in Bh, there is very weak and temporary support for Middleman/Padrino… I just added as a test.

However, I've been looking at their code, and I think I might able to fully support it (maybe in version 1.2?).

Supporting Padrino will mostly require to rewrite the internals of the methods that accept either a content or a block.

Therefore, I'm going to close this issue. What you wrote makes sense, but I want all those methods that take either a content or a block to be uniform, so I'll deal with that in a separate commit! Thanks :)