SQL databases match nullability to datashape.Option

We now inspect database columns for the `NOT NULL` attribute and use
that to determine if the analagous datashape should have Options or not

We hook this into the datashape-sqlalchemy converters and use
the database reflection built into SQLAlchemy
Merge pull request #413 from mrocklin/sql-nullable-schema

SQL databases match nullability to datashape.Option