Refactor Breadcrumb class

- Removes legacy support for making the last crumb a link
- Removes unused `reset()` and `last()` methods
- Adds `CSS` classes and `nav` element
- Ensures each title is always wrapped in an `itemprop` span
- Modernizes, clarifies, and optimizes functionality
- Adds unit test
- Updates template

Note:  This class violates the [Single Responsibility Principle (SRP)](http://en.wikipedia.org/wiki/Single_responsibility_principle) in that it both collects breadcrumb links and renders them.  These two concerns should be kept separate.