Switch to MSDelta for binary diff/patch

This PR forgets all about bsdiff/bspatch, and switches us to DeltaCompressionDotNet, which uses mspatchc.dll, the delta compression API that Windows uses.

### Here Be Dragons

This patch system is obviously **incompatible** with the previous one! To migrate, you'll have to:

1. Ship your next update that has this change, in as a *Full Update*.
1. Make sure in that full update, to ship Squirrel.exe in the update itself (to update the updater).

## Hey, it's like you just fixed all the bugs in one PR!

Yup.

* Don't try to kick out our own installed version out from under us while updating
* Don't include a bunch of .shasum files in the final generated package 
* Clean the package directory once we finish so we don't leak space.

This PR is a bit more conservative, in that it will always end up leaving one previous version in the app directory, but we don't have a super reliable way to detect when an existing app's directory is no longer being used :-/

Fixes #133, fixes #116

## TODO:

- [x] Fix broken tests that rely on fixtures generated by the old system