[FEATURE] ViewHelper Debug ViewHelper

ViewHelper can be used to debug other view helpers to read their argument names and descriptions, and the arguments you provided.

Example:

```
{someArray -> v:iterator.first() -> v:debug()}
OR:
<v:debug>{someArray -> v:iterator.first()}</v:debug>
OR:
<v:debug>
{v:random.number()}
{v:random.string()}
{v:page.absoluteUrl()}
...etc
</v:debug>
```

Outputs, for each child ViewHelper ``v:debug`` gets as tag content:

- VH doc comment
- list of allowed arguments
- list of argument values (the "final" values given to the VH)
- closer description of ``render()`` function

You can "debug" any ViewHelper this way - ones from the core, from VHS or any other package. You can use it to ensure that the arguments you are passing are getting the correct values after Fluid transforms them to whichever value type you required. And of course it allows a very quick reference to see possible arguments when you don't have any other documentation or don't know which version to look up in docs.
Merge pull request #405 from NamelessCoder/debug

[FEATURE] ViewHelper Debug ViewHelper
[BUGFIX] Fix null $rows
Merge pull request #406 from hmspl/abstract

[BUGFIX] Fix null $rows