Added ability to use a custom field name for optimistic locking

[sumocoder]

I added the option to define a custom field name for optimistic locking.
optimistic_lock_field :some_custom_field

You can still use the old way
field lock_version

If both lock_version and a custom lock field is set, It will make use of the custom lock field

[loganb]

Hello,

Thank you for this improvement! Would you mind making the interface conform to ActiveRecord's API (http://api.rubyonrails.org/classes/ActiveRecord/Locking/Optimistic.html)? I'd like Dynamoid to conform to ActiveRecord as much as reasonably possible.

[sumocoder]

Okay thanks! I'll make the change.