Add Errors model and populate on all create/update methods

Closes aptible/aptible-api-ruby#7
Merge pull request #3 from fancyremarker/errors-on-model

Add Errors model and populate on all create/update methods
Add field definitions (see aptible/aptible-api-ruby#10)
Merge pull request #4 from fancyremarker/typed-fields

Add field definitions (see aptible/aptible-api-ruby#10)