cubicweb-timesheet #3784594 add jsonp entry point to get day types [validation pending]

Backport code used by shingouz.

gingouz expects the following URL:

http://<base-url>/view?vid=daytypes.json&start=2014-04-28&stop=2014-05-04&login=somelogin

to respond something like:

[['2014-04-28', 'dt_working'],
 ...
 ['2014-05-04', 'dt_nonworking'],
]
prioritynormal
typeenhancement
done in0.11.0
load0.500
load left0.000
closed by#ede5240e460f provide a simple json(p) entry point to get day types (closes #3784594)
patchprovide a simple json(p) entry point to get day types (closes #3784594) [applied]