Bugfix: Fully clearing cache on doFlush()

Second parameter of xcache_clear_cache function indicates which chunk of cache to clear. Since doFlush() should clear entire cache the second parameter should be either omitted or set to -1 (http://xcache.lighttpd.net/wiki/XcacheApi)
Merge pull request #49 from klandaika/patch-1

Bugfix: Fully clearing cache on doFlush()