Make `CustomForeignKeyField` importable, fix migrations with `CustomForeignKeyField`

As described in #124 (and earlier in #120 and possibly others) dynamic field definition is not supported in 1.7 migrations.

@treyhunner, it looks like you were the last one to change the foreign key code, does this look okay to you?

It doesn't seem like much is to be gained from making the class definition dynamic anyway.

Fixes #124