Don't pin specific version dependencies by default.

Also, avoid depending on ez_setup:  use it at setup time only if
setuptools is not already importable.
Break out non-core dependencies.

'cli' extra maps dependencies needed by the CLI.

'testing' extra maps dependencies needed by tests.
Ensure tox passes.
Merge pull request #9 from tseaver/dependency_cleanup

Don't pin specific version dependencies by default.