Add all current Google robot user agents

This adds all the current Google robot user agents. This adds the
detection of the Google AdSense robot (Mediapartners-Google) as well.
Merge pull request #63 from dougwilson/feature/google-robots-update

Add all current Google robot user agents
Adds Test::CPAN::Changes to dzil.
Updates README.
Bumps version to 1.55.
v1.55

    - Add all current Google robot user agents (Douglas Christopher Wilson)
Adds experimental and undocumented robot_name method.
Bumps version to 1.56
v1.56

    - Adds experimental and undocumented robot_name method.
Return once the correct browser string has been found.
Return once the correct browser string has been found.
Return robot name as browser_string if browser == bot.
Document the method to browser_name madness.
Tidy everything.
Bumps version to 1.57
v1.57

    - Make internals a little less quirky.
    - Stop returning explicit undef for browser_string and os_string.  (Why
      would you ever call these in list context?)
    - Return name of robot for browser_name when browser is, in fact, a bot.
      Previous behaviour was to return undef or something just really wrong.
Stop calling the user_agent method internally.

Also, remove documentation that encourages people to pass args to the
user_agent() method.
Fixes bug where Yahoo! Slurp reported as Firefox.

Broke down the tests a bit to make them a little more manageable.
Return better names for Googlebot News, Images and Video.
Bumps version to 1.58.