Making options argument optional per docs

If options are not passed to the access-token.findForRequest() function, then the callback argument will be undefined.  This makes the options truly optional as the docs prescribe.
@bajtos I agree that the structure you suggest is very common, and in fact is the way that I initially implemented it!  But then I thought it would be good to search through the existing LoobBack code, and this is the structure that is used throughout the existing codebase - so this is what I submitted.  
But this is a case of following convention - that is, I agree that the way you suggest would be faster, and is easier to read.  Any idea of the benefit of the "loopback way"?