Improve file system support and mocks. #42
Description
At the moment we mock File Objects, but that does not work if somewhere in the code a construct like new File(mockedFile, "subdir") is used. On the other hand, there is this obscure FileSystemHelper class which does things like converting absolute to relative pathes.
After migrating to Java 7 (#33) we should migrate the file handling to Javas Path API and investigate how Projects like jimfs (https://github.com/google/jimfs) can help us to mock the filesystem.