Misplaced SQLAlchemy requirement specification

[jkankiewicz]

Being that just importing the package causes an ImportErrorto be raised, shouldn't its 'sqlalchemy' requirement be included in the install_requires keyword argument's tuple value instead of within the extras_require keyword argument's dict value?

$ virtualenv venv
$ source venv/bin/activate
$ pip install shield
$ python -c "import shield"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File ".../venv/lib/python2.7/site-packages/shield/__init__.py", line 4, in <module>
    from .decorators import rule, deferred_rule
  File ".../venv/lib/python2.7/site-packages/shield/decorators.py", line 5, in <module>
    import sqlalchemy as sa
ImportError: No module named sqlalchemy