Be more conservative with undef_object in Solution and Vocabulary. This addresses issue #108
Fixed Format.reader_symbols/writer_symbols to not load every reader/writer.
Added Format.reader_types/writer_types to get content types associated with available readers/writers.
Update File.open_file to use Format.reader_types
Include */*;q=0.1 as catch-all in Accept header.
This addresses issue #107
Implemented Value#term? and Term#term?. Added Value#variable? to replace Resource#variable?
Added Enumerable#to_{writer} and Mutable#from_{reader} behavior in method_missing based on all available Readers/Writers. This addresses issue #105