changing autoloader to use classmap

Since the file and class definition structure do not meet psr-0 standards, using a classmap for autoloading is a better option. With this change, people using the composer autoloader can instantiate sub classes, ex: Google_Service_Directory_User without having to previously included the Google/Service/Directory file.

The composer.json definition still has the include-path directive though because many class files have specific require statements in them, so they themselves do not use autoloading.

This pull request is a fix for issue #20