Added character escaping on display names

This is originally from @AdamChandler, just splitting his original PR for him. https://github.com/RustOxide/Oxide/pull/24

Some users on the forums have been experiencing problems with the function: rust.FindNetUsersByName
It would not return the correct user if the user had special characters in their name.
This will escape the special characters for pattern matching which were causing problems.
Reports:
http://forum.rustoxide.com/threads/rust-findnetusersbyname-bug-with-brackets.2279/#post-28346
http://forum.rustoxide.com/threads/passing-netuser-displayname-to-another-plugin.2404/#post-30377