Added dists and utils to Nengo namespace

As per the discussion in #548.

- Moved `nengo/utils/distributions.py` -> `nengo/dists.py`, which required a bunch of changes in import statements
- Imported things in `nengo/utils/__init__.py`, and imported utils in `nengo/__init__.py`, which should remove the need for some import statements in model code
  - This necessitated changing some imports in utils from absolute to relative. Not sure if this is an improvement...