Don't transform artist lifespan dates into \DateTime

Some artists begin and end date are limited to only a year which breaks the constructor

Merge branch 'release/v0.2'

* release/v0.2:
  Bump version number on missed user agent string
  Add some comments and code cleanup
  Add PhpUnit to composer.json and update Guzzle version
  Rename Http adapters
  Abstraction on HTTP clients and add Requests client support (http://requests.ryanmccue.info/)
  Added some comments and fixed some bugs
  Remove extra colon and fix @return tag
  Add a more in-depth example
  Cleaned up the examples and added some comments
  Wrap a query parameter in brackets if a space is present
  Add helper methods, code cleanup
  Remove duplicate require statement in composer.json
  Bump version number
  Update contact information

[composer] update branch alias