ENH: Include the 1slice example dataset in openfmri.org layout

This is just the start. The 25mm version can be added as an additional
flavor. The reason for doing this is to have a dataset that isn't
concatenated runs, but has them separate as most real-world datasets do.
Second, this is to promote the idea of having data organized according
to the openfmri scheme, including a specification of the experiment
design.
NF: Helpers to compose PyMVPA datasets form openfmri.org formated data

Adjusted a few examples and unit tests to use the openfmri-style example
dataset.

A next step, after some feedback, would be to transform the 25mm dataset
too.
ENH: OpenFMRIDataset handler gets more functionality and tests
RF: Dissolve tutorial_25mm dataset and merge with openfmri structure

Adjust tests and utility functions to use the new location.
BF: Need an empty folder for test condition.
RF: Generic event2sample_attr()
DOC: Starting to modify the tutorial for openfmri
RF: Adjust tutorial startup script (no results in latest data tarball)
DOC: Adjust mappers tutorial to use openfmri-based dataset
RF: Adjust tutorial data_generator helper to use openfmri dataset
RF: Update example to pull anatomical file from openfmri dataset
RF: Use helper functions to load example data

This also limits the direct use of attributes.txt and should help to rip
it out later more easily.
RF: Use helper for openfmri key conversion
RF: Dedicated function to get info on model conditions
BF: Support partial condition info per run
NF: Convert event lists to sample attr with evds() API
NF: Load entire openfmri dataset with a single call
RF: Adjust for new openfmri dataset input.
BF: dataset had `None` labels; not ready for that yet
RF: Move openfmri into dataset.sources