org-agenda.el: Implement new sorting strategies
* org-agenda.el (org-agenda-sorting-strategy): Document the
new sorting strategies.
(org-agenda-get-todos, org-agenda-get-timestamps)
(org-agenda-get-deadlines, org-agenda-get-scheduled): Add a
`ts-date' text property with scheduled, deadline or timetamp
date.
(org-cmp-ts): New function to compare timestamps.
(org-em): Add a docstring.
(org-entries-lessp): Use `org-cmp-ts' to compare timestamps.
Implement the following sorting strategies: timestamp-up/down,
scheduled-up/down, deadline-up/down, ts-up/down (for active
timestamps) and tsia-up/down (for inactive timestamps.)