Support image and file fields

Indeed, there is already basic support for the file entity itself though I didn't test that much yet.

I took a stab on that and it turned out being more difficult/complex than necessary. So I had a look at improving the metadata wrappers and simplified the handling of nested data structures, as we have them for fields that are composed of multiple properties (e.g. file + description).

Metadata wrapper changes:
* value() now doesn't throw an exception for not set properties any more, but returns NULL.
* Fixed dealing with arrays as data structures without the necessity of wrapping it into another object.
* Added support to auto-create data structures to ease adding stuff, e.g. you can add a file now just by doing $node->field_file[42]['file'] = $file;.

Attached patch contains the metadata wrapper improvements + implements file field support.

I've completed the patch to also add support for image fields + committed it.

based on the improvements described in #3, I also improved the text-field integration to support summaries as well as text fields with summary without text processing + added tests for that.