Issue #10, Issue #11, Create a buffer for key exchange

When initiating a key exchange, all user notifications are displayed in
a new buffer called *ircrypt. This patch introduces the basic functions
for dealing with this buffer:

  ircrypt_get_buffer        Get the buffer or create it if non-existent
  ircrypt_buffer_input_cb   Handle inputs
  ircrypt_buffer_close_cb   Removed the buffer pointer if closed

Signed-off-by: Lars Kiesow <lkiesow@uos.de>