Added a check to make sure a given attribute name won't overwrite an existing instance method

I ran into some trouble while modeling a dataset that included an "attributes" property on one of the models. See the following:
```ruby
class BadModel < Ohm::Model
  attribute :attributes
end
```
This causes `Ohm::Model.attribute` to overwrite the `Ohm::Model#attributes` method and breaks the model. I just added a check on `Ohm::Model.attribute` that throws an error if your attribute name would overwrite an instance method.