Add requirements

Signed-off-by: Gregor Best <gbe@unobtanium.de>
Rename XHR to JHR

Signed-off-by: Gregor Best <gbe@unobtanium.de>
Add 'onload' template property

Signed-off-by: Gregor Best <gbe@unobtanium.de>
Add 'headerscripts' property for injecting JS into HTML head

Signed-off-by: Gregor Best <gbe@unobtanium.de>