Option for normalization in mgmotionaverage

[alexarje]

Now normalization is on per default in mgmotionaverage. This makes sense for most things, but it would be good to have an option for turning off normalization.

[emoninet2]

feature added in #23

[alexarje]

Cool. But getting an error when trying to use it:

mgmotionaverage('dance.avi','color','normalize');

results in this error:

Index exceeds the number of array elements (3).

Error in mgmotionaverage (line 73)
if (strcmpi(varargin{argi+1},'off'))

[emoninet2]

Fixed in #25