Added dists and utils to Nengo namespace

As per the will of #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...