Add `find` method to `ReadModel`

This adds a `find` method to `ReadModel`. This will allow querying
for a specific record in the collection. It will use
ActiveRecord's `find` method on the relation. The relation itself
is only built from a subset of the read model's expressions. It
will not use any `where`, `order`, `limit` or `offset` expressions.
This is because `order` doesn't matter when selecting a specific
record and `where`, `limit` and `offset` can remove the record from
the results.

Closes #142
Merge pull request #162 from redding/jcr-rm-add-find

Add `find` method to `ReadModel`
Have `FakeRecord` use active record's default timezone for timestamps

This updates how `FakeRecord` sets timestamps to better mimic
how ActiveRecord handles it. It will now look at the
`default_timezone` configured for ActiveRecord and use that to
either build a local time or a utc time. This is what ActiveRecord
itself does.

This fixes issues with the time zones not matching up between
the timestamps `FakeRecord` set and time/datetime values generated
by `MR::Factory`. Since `MR::Factory` uses ActiveRecord to typecast
datetime values, it was already using ActiveRecord's
`default_timezone`. Prevously, `FakeRecord` was always using
`Time.now` (which is always local time) which would lead to
mismatched time zones if the `default_timezone` was set to UTC.

This fixes #158 by ensuring both sides using ActiveRecord's
`default_timezone`. The proposed solution of #158 doesn't
actually work as ActiveRecord doesn't honor the time zone in a
datetime string. This is because ActiveRecord's typecasting
expects the values to be coming from a DB that it has managed.
Thus it handles writing them using the `default_timezone` and
expects to read them as if they used the `default_timezone`.
This makes sense because many DBs don't even support a timezone
for datetime values.

Closes #158
Merge pull request #163 from redding/jcr-fix-time-and-datetime-generation

Have `FakeRecord` use active record's default timezone for timestamps