Breadcrumbs methods

Add additional methods for working with the @breadcrumbs hash. (Trying to keep implementation details encapsulated within this gem.)
* `clear_breadcrumbs`
* `breadcrumbs?`
* `breadcrumb_names`
* `last_breadcrumb_name`

Updated README to reflect these methods. Also added details about the ability to pass a block to the `render_breadcrumbs` method.

Added tests for the breadcrumbs helper and controller methods.