Implement the field attach functions for bundles

Reviewing the CCLI code for other differences in conjunction with #1369570: How is this module different from Commerce Custom Line Items (Types)?, I did notice two things that need to be done here:

1. Add validation to prevent the same line item type from being used twice. (Actually, the machine-name widget itself already takes care of this. No need to add it ourselves.)
2. Notify the field system properly w/ field_attach_create_bundle() / field_attach_delete_bundle(). I didn't even know these existed, and I'm guessing we'll need to update the core Product UI module to use these as well.