Moved test files to appropriate packages

- Using the convention of numpy, scipy, etc. in which every
  package contains a `tests` subpackage with all the tests.
- py.test finds these test without a problem, but had to add a
  conftest.py in these subdirectories that imports
  `nengo.tests.conftest`.
- Merged `nengo/tests/README.md` with `README.md`.
Moved logging stuff to nengo/utils

- Also added some unit tests for it
- Fixed a bug where you would get multiple handlers if
  you called log multiple times