Last breadcrumb set as not linked

Added "can_link" param to verify if current breadcrumb must be linked (see methods: crumb_to_html, crumb_to_html_list).

I've done it because in my web project last breadcrumb is connected with visited page. In this case breadcrumb should not be linked.

Well, it's not perfect solution. I would like to have method that checks this condition automatically with using "current_page?" helper method.

It's hard to do now because context is required. For details how to do that see "breadcrumbs_on_rails" gem.

Thank you.
adding option :last_crumb_linked with default value true for the new feature; adding suitable tests

Adding new feature - optional not linking last breadcrunb.
Added optiom :last_crumb_linkded:
 - default value: "true"
Added suitable tests:
 - to be support backwards compatibility for existing users
 - for testing new option :last_crumb_linked
Merge pull request #39 from adam-pl/patch-1

Last breadcrumb set as not linked