CSVSampleLoaders as transformers

Made CSVSampleLoaders as scikit-learn transformers

This is a good idea indeed.

I made to classes. The CSVToSampleLoader converts one line to one sample; and AnnotationsLoader that aggregates from CSVToSampleLoader to read annotations using bob.db.base.read_anno.... This is delayed.

I'm already porting this stuff on bob.bio.base. Code is way more cleaner.

ping @amohammadi @ydayer

Closes #30 (closed)