Enable xero.module to work with memcache.module

xero.module's xero_get_cache() calls cache_get(…, 'cache'), meaning that it caches data in rows in the {cache} table. With memcache.module, it's common practice to assign the {cache} table to be handled by Memcache.

Drupal core's implementation of cache_get() returns 0 for nonexistent entries.

But Memcache's implementation of cache_get() returns false for nonexistent entries. Since this isn't a number, xero.module's xero_get_cache() assumes it's cached data, and returns it, never calling Xero to populate the cache.