#8212 -  Convert sqm breadcrumb into a has_breadcrumb gem

This pull request adds test coverage to the has_breadcrumb gem.  In addition to the specs, the majority of code added involved building a test environment which included its own models, schema, db adapter and specs.

Lastly, initialization code was added to a railtie class which loads up the has_breadcrumb modules before app initialization.  Also the README was slightly edited to be more readable on what the gem does.

[#8212](https://squaremouth.lighthouseapp.com/projects/63460/tickets/8212-convert-sqm-breadcrumb-into-a-gem)