json_int_t is typedef'd to long long if it's supported, or long
otherwise. There's also some supporting things, like the
JSON_INTEGER_FORMAT macro that expands to the printf() conversion
specifier that corresponds to json_int_t's actual type.
This is a backwards incompatible change.
Couple some string and number information from the RFC conformance
chapter in the API reference, and refer to the RFC conformance chapter
from API reference for more information.
Also, state more clearly that a JSON text must have an array or object
as the top-level value, and better document the string comparison
performed by json_equal().
