Camera FileReader driver reads timestamps from filenames

The camera FileReader driver tries to parse a timestamp from the
filenames read. If possible, it is set to the corresponding
pb::ImageMsg. Otherwise, the frame number is used (current
behavior).
Code format and example of timestamps added
Path is now skipped in function GetTimestamp

Code changed to reduce complexity
Merge pull request #8 from arpg/camera-driver-timestamp

Camera FileReader driver reads timestamps from filenames