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.