General cleanup

Made some changes to improve the modules

- Changed the path functions to say _path instead of _uri. We're not
defining URIs, they are really relative paths. From discussion with
Egypt, I tried using an URI object. But it doesn't handle relative paths
at all.
- Making the file in which we plant the backdoor dynamic. Mainly because
it may improve exploitability in cases where a funky theme that doesn't
have a header.php!
- Fixing the actual exploit call for wp_duplicator_exec. I seem to have
had a bit too much fun with my CTRL-Z last night!
- Adding better error handling for the exploit.