[MERGE] Tweaks for merge directives

[John Arbash Meinel]

John Arbash Meinel has voted +1.
Status is now: Approved
Comment:
This doesn't do partial match checking (so if there is a merge-directive 
v2 it will raise NotAMergeDirective, rather than something like 
BundleNotSupported.

However, it is better than what we have, so feel free to merge.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C45F42BE5.40105%40utoronto.ca%3E