added a loader interface for file formats

Resolves-bug: https://github.com/sebhtml/ray/issues/16
Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
SequencesLoader: all supported formats use the interface

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>