Add `MR::FakeRecord::Attributes` mixin

This adds a attributes mixin for fake records that contains all
the "attribute" related logic for fake records. This is to better
organize the behavior of fake records and make it easier to test.
This is an internal change and doesn't make any changes to the
fake record's behavior yet.

This also removed an unused method from the `Model::Fields` mixin.
I noticed this because the `Attributes` mixin is similar to the
`Fields` mixin and I was using it for reference.
Merge pull request #118 from redding/jcr-split-fr-attributes

Add `MR::FakeRecord::Attributes` mixin
Add `MR::FakeRecord::Associations` mixin

This adds an associations mixin for fake records that contains all
the "association" related logic for fake records. This is to better
organize the behavior of fake records and make it easier to test.
This is an internal change and doesn't make any changes to the
fake record's behavior yet.

Overall, the associations were redesigned to more properly mimic
ActiveRecord. This meant separating out an association and
reflection class. These changes also worked to make the
associations logic maintainable and expandable for future needs.

Closes #68
Add `MR::FakeRecord::Persistence` mixin

This adds a mixin for fake records that contains all the
"persistence" related logic for fake records. This is to better
organize the behavior of fake records and make it easier to test.
This is an internal change and doesn't make any changes to the
fake record's behavior yet.

This also requires thread in the factory. This should've been
done since it uses `Mutex` for locking.
Merge pull request #147 from redding/jcr-split-fr-associations

Add `MR::FakeRecord::Associations` mixin
Merge pull request #148 from redding/jcr-split-fr-persistence

Add `MR::FakeRecord::Persistence` mixin
Fix `FakeRecord::Associations` mixin for polymorphic

This fixes the implementation of associations by ensuring an
association knows it's fake record. This allows polymorphic
associations to read the foreign type and constantize it. In MR,
this is needed by record factories and stacks to properly build
associations as needed.

The previous implementation built reflections from associations,
when ActiveRecord actually does it the other way. Furthermore,
since an association needs to know it's fake record, it has to
be lazily-built when the `association` instance method is called.
Thus, reflections are built when an association method
(`belongs_to`) is called and associations are built from
reflections when `association` is called.
Merge pull request #149 from redding/jcr-split-fr-fix-associations

Fix `FakeRecord::Associations` mixin for polymorphic