Changes to satisfy IETF Last Call comments

Add text in Section 5.1 requiring bit lengths a and k to have the same value for all MAP CEs sharing the same address, and showing that this leads to non-overlapping port sets.
Fix error in interval between contiguous ranges for specific port set noted by Lei Yan.
Add reference for Address-Dependent filtering in Security Considerations section.
Add explanation of non-overlapping port sets in Appendix B.
Add Brian Weis and Lei Yan to Acknowledgements
Merge pull request #9 from TomT5454/patch-6

Changes to satisfy IETF Last Call comments