It's now an error to try to add an object or array to itself. The
encoder checks for circular references and fails with an error status
if one is detected.
Added functions:
json_string_set
json_integer_set
json_real_set
While at it, clarify the documentation and parameter naming of
json_{string,integer,real}_value() a bit.
All pointer arguments are now tested for NULL. json_string() now also
tests that strdup() succeeds. This is to ensure that no NULL values
end up in data structures.
Also desribe the different sources of errors in documentation.
