WIP: A first unit test for NI-DM export

This is a proposal to write unit testing procedures for NI-DM.

A single test (`test01`) is currently implemented. My intention is to have several of these tests covering different types of analysis (1st level  model/ 2nd level model, with / without conjunctions, including 1 / several contrasts…). This version is only a first draft but I wanted to solicitate your input. In particular, do you think that this kind of tests would indeed be useful and support us in the validation of NI-DM export tools? If so, I would also be happy to get some feedback or comments on this draft.

I think that this topic was previously mentioned (in particular with @nicholsn and @satra under issue #9 and PR #6) and any input would be greatly appreciated.

In short, for each test, the testing function checks that the provn file produced by the analysis software (SPM, FSL) is compliant with a gold standard (written manually based on the data model).

`Test01` investigates a single-subject fMRI analysis of [auditory data available on the SPM website](http://www.fil.ion.ucl.ac.uk/spm/data/auditory/) described in the script `test01_spm_batch.m`. The expected output (created manually) is stored in `test/spm/GroundTruth/test01`. The provn file generated by spm is to be stored under `test/spm/SPMexport`.