Currently importing yamllint recursively imports its submodules, which
finally requires having pyyaml installed. This is a problem when you
just want to import APP_VERSION from yamllint. For instance, setup.py
imports yamllint to know the version, but doesn't know yet that pyyaml
is to be installed, because it is stated in setup.py itself.
To solve this, yamllint/__init__.py will only contain constants. The
linting functions will be in yamllint/linter.py.
