org-habit.el: New file, which implements code to build a "habit
consistency graph".
org-agenda.el (org-agenda-get-deadlines)
(org-agenda-get-scheduled): Display consistency graphs when outputting
habits into the agenda. The graphs are always relative to the current
time.
(org-format-agenda-item): Added new parameter `habitp', which indicates
whether we are formatting a habit or not. Do not display "extra"
leading information if habitp is true.
