Merge pull request #17 from playcanvas/master

merge
Point light shadows. There could be some problems on Intel videocards (?) and old Macs (?);
Falloff mode as light property;
Inverse squared falloff mode.
Minor fixes
Fixed falloffMode not recompiling shaders on change;
Added docs for falloffMode.
doc fix
Merge pull request #75 from guycalledfrank/point-shadows-falloff

Point shadows + falloff