Expand `ReadModel::Querying` to include additional query methods

This expands the querying mixin to provide methods for most of
ActiveRecord's relation methods. This allows advanced SQL queries
for pulling complex data. This should allow read models to be
flexible for whatever specific data is required and make use of
ActiveRecord's querying features.

Related to #88
Merge pull request #103 from redding/jcr-read-model-expand-querying

Expand `ReadModel::Querying` to include additional query methods