Adds output of files container-format

This adds a new array-entry to the fileInformation-array that contains
the container-format of the file.

This has been testet with ffmpeg (version 1.2.4) and avconv (version
0.8.6-6:0.8.6-1)
Fixes return values

The method of course should return a string and not a boolean value
Merge pull request #13 from heiglandreas/feature/addsContainerFormat

Adds output of files container-format