Autoload function for Google_* classes

Addresses #341
Use autoloader instead of requiring class files

For backwards compatibility (if someone still includes a class file
directly instead of using the autoloader), we make sure to always use
the autoloader whenever appropriate.

Addresses #341
Update README.md and composer.json

Addresses #341
Fix case of Google_Cache_Apc in CacheTest.php

Addresses #341
Merge pull request #342 from ltrebing/autoload

Use autoloading instead of include path