Support for WordPress Networks

I've downloaded your plugin as an alternative to WP Super Cache. However, I found there was an issue with WordPress Network installations. Cached files are currently saved into one folder "/cache/all" which works for individual sites but causes incorrect results for sites within a network.

If you use this plugin in a WordPress Network, all sites will serve the same content of the first cached site. Instead of opening a support ticket, I decided to improve your plugin to support networks as well as keeping the current functionality for single installations.

I'm submitting this pull request so you can review it, test it and improve it if needed. If you want to check how does it work, please feel free to visit my WordPress Network where this (improved version) is working: http://en.8elite.com http://es.8elite.com

This has been tested in a single WordPress installation as well as a WordPress Network with subdomains activated. I didn't test it in a WordPress Network with subfolders, however, in theory it should work. Nonetheless, I encourage you to test it and make sure it works before merging this into your master branch.

I'm attaching two screenshots from those sites to prove cached files are being correctly served.
![es8elitecom-cached](https://f.cloud.github.com/assets/2735947/1949734/544f3f12-811d-11e3-8385-d4c697b97d67.png)
![en8elitecom-cached](https://f.cloud.github.com/assets/2735947/1949735/54577bc8-811d-11e3-92e3-59547f82b12e.png)