Made EUTI API a bit more consistent. Initializers or factory methods whose last method keyword is 'supertypes:' were taking an array of UTI string representations, when -supertypes returns an array of ETUTI objects. Now 'supertypes' in a method name always means an array of ETUTI objects, and 'supertypeStrings' an array of UTI string representations.

Added a new method -transientTypeWithSupertypes: (useful for EtoileUI implementation). The previously identically named method has been renamed -transientTypeWithSupertypeStrings:. 

Modified documentation to make clear that methods such as -supertypes, -subtypes that return UTIs won't return string representations but ETUTI objects.

Updated test_ETUTI.m as needed and checked that all test pass.