WP CLI integration

I would like to ship some WP CLI commands with this library. I would like to provide the following:

- [x] Easy installation (requires closing #26)

 ```bash
wp plugin install whatever-i-title-this-thing
wp memcached install
 ```
 
 This would handle moving/symlinking the file into place and adding the basic config to wp-config.php.
- [x] Test configuration

 Add a command to test if certain parts of the configuration are set correctly. It could check for:
 - [x] PECL memcached availability
 - [x] Memcached daemon availability
 - [x] Memcached is available from PHP
 - [x] Cache can store content

- [x] Stats - dump basic Memcached stats via the nicely formatting WP CLI tables

- [ ] ~~Check for the [go-derper](http://www.sensepost.com/blog/4873.html) vulnerability (i.e., make sure 11211 or whatever port is being used is closed)~~

- [x] Add comments

- [x] Add documentation

- [x] Add testing