Update README with ruby 2.0 compatibility.
Add Format.file_extension to return format-specific file extensions. Added specs for this and for .content_type. This addesses issue #16.
Use headers passed to Reader.open, and then to Util::File.open_file when calling Kernel.open (OpenURI.open_file).
Infer the Accept header, if not provided, from loaded readers.

This addesses #106.