Add custom Preposition field to Activity Stream with backwards compatability

In our use case, the hardcoded 'on' preposition is less than ideal.  We have users who can create entries for challenges, resulting in the following action:

r3dfish created My Entry on Challenge of the Century

I have modified the codebase to allow defining of custom prepositions when registering the action, resulting in the following action:

r3dfish created My Entry for the Challenge of the Century

This code is also backwards compatible, so if you do not define a custom preposition, the built in 'on' will be used as a default.