Add test suite and fix a couple of bugs

* Support WordPress Multisite setup
* Add PHPUnit test to ensure compatibility
* die() when people mistakenly try to activate this as a plugin, and provide a helpful message for where they should move the file. (from APC backend)
* Fix incr() and decr() functions used. This fixes issues with BatCache and other plugins.
* Fix $force option in get() function
* Fix legacy use of reset() function


git-svn-id: https://plugins.svn.wordpress.org/xcache/trunk@794365 b8457f37-d9ea-0310-8a92-e5e31aec5664