Lua User Store #11

User store implemented as a Lua module.

Parameters are:

file: Path to the Lua file.
params: Variables to be passed into Lua.

The Lua module will implement the following, which directly correspond to the UserStore trait:

get_user(username): return a User table, or nil
login(username, password): login the user, return a User table if success, nil on error
registration_available: return true if registration is available, false otherwise. Default implementation will return true if the register method is available.
register(username, password): registers a user, returns User struct if success, nil on error.

This will most likely supplant #8 (and maybe even #9 and #10 which themselves could have been implemented with #8) since the Lua module can massage the return value of the command (or web service etc) into a user struct.