Add a version_info attribute

This makes comparing docker-py versions *much* easier. Having to account
for the potential of a "-dev" suffix is a PITA.

docker/__init__.py

@@ -12,7 +12,7 @@
 #    See the License for the specific language governing permissions and
 #    limitations under the License.
 
-from .version import version
+from .version import version, version_info
 
 __version__ = version
 __title__ = 'docker-py'

docker/version.py

 version = "1.1.1-dev"
+version_info = (1, 1, 1)