Improve the detection of missing required fields.

Now the limit of tracked fields is configurable at compile-time using
PB_MAX_REQUIRED_FIELDS. Added related test and updated documentation.

Fixes issue #18.

pb.h

@@ -22,6 +22,12 @@
 #define UNUSED(x) (void)(x)
 #endif
 
+/* Number of required fields to keep track of
+ * (change here or on compiler command line). */
+#ifndef PB_MAX_REQUIRED_FIELDS
+#define PB_MAX_REQUIRED_FIELDS 64
+#endif
+
 /* List of possible field types. These are used in the autogenerated code.
  * Least-significant 4 bits tell the scalar type
  * Most-significant 4 bits specify repeated/required/packed etc.