Add basic foreign key storage/validation to Table.
Fix incorrect defaults.
Implement foreign key generation in Sqlite

Add the various kinds of foreign keys that Sqlite can generate.
Namespace table so it doesn't conflict with standard fixtures.
Add stub methods for describing foreign keys to all schema classes.

Describing foreign keys often requires separate logic from indexes. Add
skeleton methods to do that work.
Add foreign key reflection to sqlite.
Fix coding style error.
Code review comments.
Rename options so they are all strings.

After some discussion we came up with better names for the constraint
options.
Use constants instead of bare strings.

Constants feel better in code vs. bare strings. They also
cause errors when you make a mistake.
Merge pull request #1333 from markstory/3.0-sqlite-fks

3.0 - Add Sqlite Foreign key support