Added a way to disable ForeignKey on historical models via 'preserve_fk' keyword

[jeanphix]

[smn]

Thanks for this!

Could you perhaps:

1. Give an example use-case of this for in the README? Just an example bit of code for how this works and why you would want it / what problem it solves?
2. Remove the .egg file from the commit? I prefer not have generated files in the repository.