initial work for #79 possibility to mock large files without large memory footprint:
   * refactored file content handling into separate class
   * deprecated org\bovigo\vfs\vfsStreamFile::readUntilEnd(), was for unit test purposes only but unfortunately part of public api
   * deprecated org\bovigo\vfs\vfsStreamFile::getBytesRead(), was for unit test purposes only but unfortunately part of public api
more work on #79: refactor seek functionality in new base class, create possibility to have large file content
provide possibility to pass any FileContent instance to a file
add integration test for large file handling
update changelog with latest changes