Refactor config module to export a lazy-load object

[berinhard]

@isabarros raised a very valid point during our discussion in the PR #93. To expand pyp5js configuration and also to be able to unit test the base SKETCHBOOK_DIR creation, we should change the existing pytest.config module to export a lazy-load object instead of giving direct access to its configuration constants. Django's settings should be the main guide as an approach to address that. The ideal import to use pyp5js configs would be:

from pyp5js.config import conf
print(conf.SKETCHBOOK_DIR)

I think it's only worthwhile to work on this issue after #93 is merged into develop. This is because this new conf object implicates in a few code changes that would be better to do with a better test coverage for the web application.