[Feature][Processing] Headless

To use QGIS Processing in python scripts out of QGIS.
The changes made concern the dependence to legendInterface. In cases
where the interface.iface is null (None) QGIS Processing uses the
QgsMapLayerRegistry instance.
Merge pull request #1031 from rldhont/headless_processing

Some changes to allow Processing to be run headless

It replaces legendInterface with QgsMapLayerRegistry, which is a better option
[processing] minor cosmetic change