Hook buffer clear commands in client

Hooks the commands /CL and /buffer clear to clear the chat window in the
android app when clearing the screen server side.  Doesn't work if you
use a different alias.

Partial fix for #9
Add tap+hold menu for copying message text

When viewing a buffer, you can now tap and hold on a line to copy the
text.  If the line contains url's, you can copy them separately as well.

Fixes #60.