Fix markdown to handle :safe, safe input and add tests

At present markdown doesn't handle the :safe option:

ruby-1.9.2-head > helper.markdown("test", :safe)
TypeError: can't convert Symbol into Integer

It also doesn't handle safe input properly (the <script> should not be escaped since the text is marked html_safe):

ruby-1.9.2-head > helper.markdown("This is worded <strong>strongly</strong><script>code!</script>".html_safe)
 => "<p>This is worded <strong>strongly</strong></p>"

I've fixed these two issues and also added tests. Patch attached.