Add support for deleting status columns. If a column is not empty then
it will not be deleted and the user will see an error message in the
status bar.
Part of apollo/pelican#24
This commit adds support for creating new status columns. When the user
is in the 'board edit' mode, they can press the 'c' key to create a new
column. When a new column is created it automatically assumes the last
position on the board. We will add support later on to allow the user to
re-arrange the columns on the board.
Part of apollo/pelican#22
