docs(ProcessInstanceAssert): document new assertions 'isWaitingFor', 'isNotWaitingFor' and helper methods 'send'

Affects #29
feat(ProcessEngineTests): create two variants of helper method 'send' to easily send messages to specific process instances

Affects #29
feat(ProcessInstanceAssert): create new assertion 'isWaitingFor' to verify process instances

Affects #29
feat(ProcessInstanceAssert): create new assertion 'isNotWaitingFor' to verify process instances

Closes #29