wikia.api: support mcache URL parameter

This should ease the debugging of Wikia API requests
Add Wikia::setUpMemcachePurge() method to be called from wikia.php
Resolved merge conflicts.
BAC-688 Pass `$user` to `Wikia::setUpMemcachePurge()`.
BAC-688 Pass `$this->wg->User` to `Wikia::setUpMemcachePurge()`.
BAC-688 Fixed typo.
Merge pull request #1276 from Wikia/wikia-api-support-mcache

wikia.api: support mcache URL parameter