Reapplied the users->user change in brendon's code

Used the regex ([uU][sS][eE][rR])([sS])([^ "][^p])
with replace \1\3

To replace all the references in the PHP to users_id and such to user
singular. Maintained references to the USERS table itself.
Updated class names to reflect the users to user rename.
Changed references to "Users ()" class and "Users" to User singular