Makes degu rails 4 compatible. Adds usage instructions

Hi Florian.

Can you please review this code? 
Three things were done so far:
* adds so called definition extensions, i.e. helper methods you can use during the definition of the enumeration to define the values of attributes. You can also define them at the beginning of the file using `def self.method`, but it can get pretty messy.
* Changes the dependency of active_record, so you can use rails 4. Also removes all the warnings generated.
* Adds documentation. Can you please read through it, eliminate all errors and add your stuff? I should probably visit some english courses :laughing:

If everything is ok, can you release the new version. By the way, `gem_hadar` is outdated. Can you update to the latest version, too? 

Greetz

Andreas