Test NIDM Dataset Descriptor at the Project Summary Level

First shot at creating a test using a sparql query to validate that the required "MUST" attributes are present in a NIDM Dataset Descriptor.

@satra, could you take a quick look at this test and see if it makes sense. Feedback also appreciated on how I've implemented query.py and utils.py. Need to add docs, but here is an example session:

```
In [1]: import niquery as niq
In [2]: select = niq.SelectQuery()
In [3]: select.sparql_meta.title
Out[3]:
0                     Select All Contrasts
1                     Select All Summaries
2                       Select All Studies
3                 Select All Distributions
4                       Select All Queries
5    NIDM Dataset Descriptor Summary-level
6                      Select All Projects
7                      Select All Versions
8                  Select All Acquisitions
Name: title, dtype: object
In [4]: result = select.execute(5, turtle_file='/Users/nolan/Repos/niquery/tests/data/nidm-descriptor.ttl')
In [5]: result.keys()
Out[5]: Index([u'uri', u'type', u'title', u'description', u'publisher', u'hasPart'], dtype='object')
```