Added typespecs on all exported functions

Made sure all exported types do have a typespec is useful in build scenarios
where the erlang compile options
[warnings_as_errors, warn_missing_spec]
are used.

A few type errors found by Dialyzer are corrected too.

	modified:   src/jsx.erl
	modified:   src/jsx_config.erl
	modified:   src/jsx_config.hrl
	modified:   src/jsx_decoder.erl
	modified:   src/jsx_parser.erl
	modified:   src/jsx_to_json.erl
	modified:   src/jsx_to_term.erl
	modified:   src/jsx_verify.erl