Included metadata during the feature extraction.

Hi guys,

I have a feature extractor that needs to use some meta information that is stored in bob.bio.base.database.BioFile.

In this MR, I added in the preprocessor.call and extractor.call a keyword called metadata that will ship an instance of bob.bio.base.database.BioFile. Some design decisions, that will may generate discussion, are commented in the MR diff. Please have a look there.

Extending this to other elements in the toolchain (enrolling, scoring), could provide a solution to !111 (closed).

@mguenther, do you have some time to review this one? Thanks in advance.