Make offline validation the default

[draeger]

JSBML's validation capabilities have greatly improved and are now very reliable. In some situations, the online validation can be problematic, e.g., if no network connection is available or when the URL pointing to the service needs to change. The latter problem could be fixed by implementing a configurable solution in an exchangeable file, but switching the default behavior of JSBML's validation to offline mode would ease many processes.