- Added an ImageSource with landmark labels (LabeledImageSource)

- added FilebasedImageSource interface as an additional layer
- added LandmarkSource class: a generic source for landmarks
- added LandmarkFileLoader class that provides different means for
loading landmark files from the disk
- added LandmarkFormatParser interface and implementations for .tlms and
.did
- rearranged some files in CMakeLists
- renamed LandmarksCollection to LandmarkCollection
- integrated FaceBoxLandmark into the Landmark class