Fix boids example and some bugs in gloo

Related to buffer when data is not contiguous
Gloo: add tests for non-contiguous data to buffer
Merge pull request #218 from almarklein/boid-fix

Fix boids example and some bugs in gloo
New demo: spacy + fixes to gloo/buffer.py

The set_data method of a DataBuffer should have offset in vertices,
not in bytes...
Gloo: tests for fixes to offset handling in DataBuffer.set_data

Also pep8
Style
Merge pull request #220 from almarklein/spacy-demo

New demo: spacy + fixes to gloo/buffer.py