Add tests to img liquid_tags

- Rename `test_notebook.py` file to `test_liquid_tags.py`
- Added `unicode-literals` in `test_liquid_tags.py` import as in pelican
tests.
- Changed attributes dictionary type  to OrderedDict in `img.py` for
  tests purposes.
  - Added 4 tests for the img tag in `test_liquid_tags.py` file

  Changes tested in python 2.7, 3.3 and 3.4.