Add basic_parser header and body limits:

fix #550

* default body limits are now 1MB/8MB
* default header limit is 8KB

Actions Required:

* Call body_limit and/or header_limit as needed to adjust the
  limits to suitable values if the defaults are insufficient.

test/http/error.cpp

@@ -42,6 +42,7 @@ public:
         check("beast.http", error::unexpected_body);
         check("beast.http", error::need_buffer);
         check("beast.http", error::buffer_overflow);
+        check("beast.http", error::body_limit);
         check("beast.http", error::bad_alloc);
 
         check("beast.http", error::bad_line_ending);