Bugfix - delta package update is failing

Resolves #170 - was a little bit more harder to nail down than I thought.

Anyway, applying multiple delta packages was crashing hard with a checksum issue.

 - preconditions highlighted that the delta package must not exist
 - we were incorrectly passing in the full file path to create a package
 - sort releases by number when writing and reading to ensure we do things In The Right Order

TODO:

 - [x] unit test for applying delta packages is all the win
 - [x] test in desktop demo app