Wand engine

Hi, I developed a new engine for sorl-thumbnail to support [Wand](http://docs.wand-py.org/en/0.3-maintenance/) ImageMagick Python binding.

### Changes

- New engine added
- Updated test runner
- Updated documentation

This implementation passes the same tests that PIL engine but in my local machine 3 tests fails:

- testUrl (thumbnail_tests.tests.TemplateTestCaseB)
- testPortrait (thumbnail_tests.tests.TemplateTestCaseB)
- test_image_file_deserialize (thumbnail_tests.tests.SimpleTestCase)