Test monetize with allow nil

Added `allow_nil` modifier for monetize specs and updated the README.

If you had specified a model with:

```Ruby
class Order < ActiveRecord::Base
  monetize :revenue_cents, allow_nil: true
end
```

then specs that specified:

```Ruby
describe Order do
  it { should monetize(:revenue) }
end
```

would fail because in the `TestHelpers` the check `actual.send(money_attr).instance_of?(Money)` responds with `false`, since the return value of `#send` is `nil`.

I also broke the single line check into multiple statements. IMHO it's a bit more readable this way (otherwise it would become really difficult to follow with the introduction of `@allow_nil`).