feat(ProcessInstanceAssert): add new assertion hasVariables

Affects #16
feat(ProcessInstanceAssert): add new assertion hasNoVariables()

Affects #16
feat(ProcessInstanceAssert): add retrieval of chained 'MapAssert<String, Object> variables()' in order to assert against the map of process variables in more detail

Closes #16
docs(ProcessInstanceAssert): add some documentation for hasVariables(), hasNoVariables(), variables()

Affects #16