Support additional test frameworks (minitest)

[mzruya]

To support additional testing frameworks, we'll have to split this gem into a few sub-gems that live in the same repository.

My current thinking is that we'll have the following:

• ar-query-matchers-rspec
  • would depend on the ar-query-matchers-core gem.
  • would have an RSpec dependency
  • in-charge of implementing and registering RSpec matchers
  • would include an RSpec integration test that uses the matchers
• ar-query-matchers-minitest
  • would depend on the ar-query-matchers-core gem.
  • would have a Minitest dependency
  • in-charge of implementing and registering Minitest matchers
  • would include an Minitest integration test that uses the matchers
• ar-query-matchers-core
  • would have no test framework dependencies
  • includes query instrumentation code. (all code currently under lib/ar_query_matchers/queries)
  • includes expectation formatting code. (all code currently inside the MatcherErrors module)

To keep backward compatibility, the ar-query-matchers gem could include ar-query-matchers-rspec as a default.