Create separate decrypt program

[pcrockett]

Currently we just copy the kin executable itself to the backup package and rename it to decrypt. When the backup recipient runs the decrypt executable, it sees its name is decrypt and just defaults to the decrypt command without any command line parameters.

It's a hack, and shouldn't stay that way. The reason is because we need to save 3 separate decrypt binaries to the backup package: one to support every major desktop OS. So having Kin copy itself to the backup package doesn't make sense. We need a separate decrypt program that gets compiled for all major OSes separately, and then Kin can copy those to the backup package.

[pcrockett]

We now have a separate single-purpose decrypt program bundled with the backup package. However we're only supporting the OS that you are currently using when building the package; we're not saving three separate executables, one for each OS.

Going to call this issue "done" and open another one for cross-platform decryption.