refactor(fluent-api): rename parallel method

Rename the parallel method to a moveToLastGateway() method which
returns a builder for the last gateway if there is only one
incoming sequence flow. The parallel method with id is now
a generice moveToNode method which returns a flow node builder
for the given id.
refactor(fluent-api): add id as optional argument for new element

You can now create elements in the fluent api with an optional
argument which is the id of the element.

For example you can replace .startEvent().id("start") by
.startEvent("start").