* exifr is used for most tags now
* New timestamp handling, removed exif-parser, date supported for png
* Removed offset from testhelper. It's optional
* explanations
* Feature/timestamps (#3)
* preparing for further timestamp test
* Added more test and fixed offset calculation bug
* Revered old dimension test, added new timestamp tests, some bug fixes
* Renamed png-test because faces overrule keywords
