Refactoring:

    - Fix bug with displaying HTTPError
    - Fix year in src files headers
    - Use ``format()`` for formatting and string concatenation
    - Rename ``get_providers`` and ``get_jobs`` to ``search_providers``
      and ``search_jobs`` for convenience
HR API Refactoring

    - Add 'Make offer' and 'End Contract' API calls
    - Refactor APIs in the "hr" namespace to be consistent
      with the Public oDesk API
Remove key-based auth functionality

    Add example for web based keys and for desktop keys,
    remove old examples
Refactor tests and fix some bugs