Command ban has been modified, tested and seems to work well.

However, the way we browse channel and client lists in the Command functions of the server class
makes the code intricate, we need something to solve that !
Addition of two new methods in the Server class providing a simpler way to browse
the client list and channel list member in the command methods of the server
which will make the code clearer.
Command nick has been modified to use the last two functions implemented (see last commit).
It seems to work well.