add private factory method to Partner

Add private factory method: Partner.find_and_instantiate in
  preparation for Partner.all implementation.
Refactored existing Partner.find to use new method.
use ```private_class_method``` in Partner class

Change to ```private_class_method``` instead of
  ```class << self; private; def ...``` to control class method
  visibility in Partner class.
add Partner.all functionality

Add examples, code, and cassette for Partner.all functionality.
Merge pull request #11 from jgpawletko/add-partner-all

Add Partner.all functionality