Changed mechanism to detect matching handlers.

The handlers used to access fields and invoke methods are now determined add object scanning time. That means we will get possible errors earlier and can also determine all those options which have no suitable handler. Previously, they were determined when the commandline was parsed.

Also the handling of final fields was changes again, fixing issue https://github.com/ToToTec/CmdOption/issues/8. Now, each CmdOptionHandler can (and must) decide if it can handle a field.

Also updated documentation.