Metadata object

Aaaaand the last important object is in!! I was struggling with a couple of errors and a merge conflict, but it is finally here!!

The whole idea of the Sample/Prep templates objects is to behave as a dict of dicts, in which the first  level of keys are the sample ids and the second one are the metadata category headers.

Anyone available to review this??
skbio.flatten would work here. But I think skbio is not yet a dependency so ok to leave as is.