Small modifications to handle exit_key set to None.

[PabloLec]

PyCUI main object exit_key can now be set to None.

What this pull request changes

Code modifications are:

• Catching None value and returning None in get_char_from_ascii() function.
• An if-statement in __init__.py to adapt _init_status_bar_text if there is no exit key.

Main draw loop has not been modified and I think this should not be a problem since default "empty" key value is 0 and not None.
I did some testing with basic examples and the key integration seems to work as expected.

Issues fixed with this pull request

• Issue Can't unset the exit key #115