Wrapping JS assets

I'd like to be able to wrap JS assets in functions to prevent global leaks/collisions. Three new property would now be supported in the assembly.json

* simpleWrap - This would wrap the file in a common function defining window and undefined.
* prefix - This would allow a developer to specify a specific prefix for this asset.
* suffix - This would allow a developer to specify a specific suffix for this asset.

These changes are additive and shouldn't effect any existing functionality.

I have added test coverage for content-resolver.