Wordpress mixin

I created a Wordpress mixin to abstract some common wordpress methods.
rspecs are not implemented since a mocking for rex sockets/http is needed.
Regexes and logic taken from https://github.com/wpscanteam/wpscan

# Changes
* New Wordpress Module
* The following modules were updated using the new module
  * auxiliary/scanner/http/wordpress_login_enum.rb
  * auxiliary/scanner/http/wordpress_pingback_access.rb
  * exploit/unix/webapp/php_wordpress_total_cache.rb
* New method `path_from_uri` in `Msf::Exploit::Remote::HttpClient` to parse an uri string and return only the path and query
* New method `peer` in `Msf::Exploit::Remote::HttpClient` used by many exploits, but defined per module
* Copy paste error in lib/msf/core/exploit/web.rb
* Checks in the changed modules to see if the provided target is online and running wordpress
* Updated the php payload in php_wordpress_total_cache.rb so there will be no error messages when the post is called via browser without the HTTP header set. This makes the exploit less noisy
* Included a random text in the php payload in php_wordpress_total_cache.rb, to check if the comment was posted correctly
* Updated php_wordpress_total_cache.rb to also work with moderated comments. Wordpress shows them only to the user who posted them (via a cookie). By setting this cookie, the payload can be executed even on moderated comments.
* php_wordpress_total_cache.rb will now try to fetch all blog posts via the rss feed and only if that fails, a bruteforce based on postids is started

# TODO:
- [x] fix indentation
- [x] Check which Wordpress Versions are supported by the module and document them 

## TODO after merge:
- [ ] consistent naming on wordpress modules - move them into wordpress subdirectory
- [ ] update the other wordpress modules to use the new mixin
- [ ] add @CharlieEriksen methods from https://github.com/rapid7/metasploit-framework/pull/1301
- [ ] remove self defined peer methods from various modules since it is now defined in HttpClient