allow issue date adjustments from gantt chart

Allows dragging issues horizontally in the gantt chart and adds buttons
to change start or due date.

This is a port of the original redmine patch: http://www.redmine.org/issues/2024
revert accidental debug log level enabling
Merge branch '1277_editable_gantt' of https://github.com/asinteg/chiliproject into thothonegan_master
Updated README