ACT-1652 Add support for message-events to REST-API

This adds support for starting processes through message events and for triggering (intermediate-) message events. This will also allow using event sub-processes through the REST-API.

The /process-instance method has an undocumented feature allowing the usage of the processDefinitionKey for starting an instance. Since I had to update this section (add the messageName parameter), I added it as well.