Initial import of tasks application

This initial import is a heavily modified version of the code I had
here, as Arse was modified for other purposes in the meantime and the
application no longer worked with it.

In addition:
 * I did not import the user management part yet,
 * task dependencies are supported in-base, but there is no interface
for that yet.

README

@@ -0,0 +1,7 @@
+Requires PostgreSQL 9.0+
+
+1) Copy/rename database/config-sample.sql to database/config.sql,
+2) Set the web app's user name and the database's name in database/config.sql,
+3) Run the database.sql script from psql,
+4) Copy/rename includes/config-sample.inc.php to includes/config.inc.php,
+5) Set user name / password in includes/config.inc.php