Allow setting IV and salt modes by Proc

Allows for setting :mode by use of proc. 
Useful for people who need to support both single and per attr modes of salt and iv
Example:
```ruby
attr_encrypted :email,  :key => SECRET_KEY, :mode => Proc.new { |obj| obj.decide_mode }
```
Should we be wrapping this in ```evaluate_attr_encrypted_option```?