Make real number encoding and decoding work under all locales

The decimal point '.' is changed to locale's decimal point
before/after JSON conversion to make C standard library's
locale-specific string conversion functions work correctly.

All the tests now call setlocale(LC_ALL, "") on startup to use the
locale set in the environment.

Fixes GH-32.

src/load.c

@@ -524,12 +524,7 @@ static int lex_scan_number(lex_t *lex, int c, json_error_t *error)
 
     lex_unget_unsave(lex, c);
 
-    saved_text = strbuffer_value(&lex->saved_text);
-    errno = 0;
-    value = strtod(saved_text, &end);
-    assert(end == saved_text + lex->saved_text.length);
-
-    if(errno == ERANGE && value != 0) {
+    if(jsonp_strtod(&lex->saved_text, &value)) {
         error_set(error, lex, "real number overflow");
         goto out;
     }