Eliminate circular import dependency

`ansible.callbacks` and `ansible.utils.display_functions` had circular dependencies.

I removed the latter dependency, because it was depending on something at a higher level which is a smell. I did this by adding a `set_display_callback` function, so that display callback can be set at runtime rather than import-time. This is also more flexible.

I also changed the imports of ansible stuff in `ansible.callbacks` from using implicit relative imports to explicit absolute imports. Explicit is better than implicit. With `import constants` or `import utils`, it's a little bit harder to tell that the module being imported is within ansible.

```
$ cat > tox.ini
# Tox (http://tox.testrun.org/) is a tool for running tests
# in multiple virtualenvs. This configuration file will run the
# test suite on all supported python versions. To use it, "pip install tox"
# and then run "tox" from this directory.

[tox]
envlist = py26, py27

[testenv]
deps =
    passlib
    PyCrypto
    nose
    PyYAML
commands =
    nosetests -d -w test/units {posargs}

$ tox
GLOB sdist-make: /Users/marca/dev/git-repos/ansible/setup.py
py26 inst-nodeps: /Users/marca/dev/git-repos/ansible/.tox/dist/ansible-1.7.zip
py26 runtests: commands[0] | nosetests -d -w test/units
.......................................................................................
...........................................................................
----------------------------------------------------------------------
Ran 162 tests in 4.803s

OK
py27 inst-nodeps: /Users/marca/dev/git-repos/ansible/.tox/dist/ansible-1.7.zip
py27 runtests: commands[0] | nosetests -d -w test/units
........................................................................................
..........................................................................
----------------------------------------------------------------------
Ran 162 tests in 4.813s

OK
____________________________________ summary _____________________________________
  py26: commands succeeded
  py27: commands succeeded
  congratulations :)
```

and here's a passing Travis CI build: https://travis-ci.org/msabramo/ansible/builds/28718413