Updated the events output to handle the following scenarios:

1) When there are only owner assertions corresponding to a package version,
don't capture version info. As a consequence, when there are only
owner assertions corresponding to 2 version of the same package, there will
only be 1 set of assertions
2) When there are only remove owner assertions corresponding to a package,
Exists is not set at all
3) When there is at least 1 add owner assertion corresponding to a package,
Exists is always set and always set to true

(2) and (3) above help process package assertions and package owner
assertions independent of each other
Added 'nupkgUrlFormat' to create a nupkg url based on package Id and version
Fixed and added tests around 'nupkg' field in the event
Made EventNewer and EventOlder relative paths
Also, added tests around it
Also, added for index.json