WIP: 0.6.9 - The One Where We Test Updating

Resolves #56 

Done

 - delta packages now use the previous full release as a base package (not the latest full release)
 - tidied up the output from CreateReleasePackage (now generates the full release and *maybe* the delta release filenames)
 - extracted filename-to-version to extension method
 - added a test to ensure no-one passes in a newer version as a baseline package
 - when installing new version over top of existing version, try and use delta packages and then fallback to full upgrade if it doesn't succeed. Resolves #70

TODO:

 - [x] tidy up messages - list packages found first, then outputs, etc
 - [ ] test real delta updates without having debugger attached
 - [ ] document how delta packages work
 - [x] #58
 - [ ] #74