[WIP] Generate schema correctly with non-default column/table names.

[grigi]

Should fix #55

I was quite surprised at how hardcoded some of the field names were, especially with foreign keys.

• Tests & fixes to generate schema with custom field/table names
  • Test custom PK name & relations from FK
  • Test source_field column with index uses correct name
  • Test custom FK name is preserved (specifically the issues that ForeignKeyField with source_field argument #55 is talking about)
• Use this model to test basic functionality works as expected:
  • Save & Simple get_by_id
  • Search by indexed non-pk column
  • Forward foreign key
  • Backward foreign keys
  • Filter by foreign key values
  • M2M keys