Task dependencies, easier folder tree traversal and checks on modifying deleted entries

We've added several new features into our API and made slight modifications to existing methods to make interactions more convenient and logical.

First of all, we now support a feature that was previously accessible only through the web-interface: task dependencies. They are mostly used on our own Timeline view (Gantt chart) but now you can make a similar representation. Dependencies are not part of a Task model but a separate entity with its own group of methods - check the Dependencies reference.

In order to facilitate folder tree traversal we've done the following:

• added childIds list to folder model
• added descendants=true/false parameter to [GET]/folders/id/folders

Also we now strictly forbid modification operations with entries which are children of deleted entries or deleted entries themselves, e.g.:

• Create/modify comment in deleted task
• Create/modify timelog in deleted task
• Add/modify attachment in deleted task
• Modify deleted task itself