dmg: fix deterministic dmg creation and docs

I've verified that the process works by manually extracting the signature from rc1 and running it through the re-attaching procedure. Updated the docs and descriptors to fix versioning. Rather than bumping the version for each build, make the inputs version-less. Obviously they must match anyway.

In case anyone wishes to test (I've verified that the dmg works fine)
The detached signature can be found here:
https://bitcoincore.org/cfields/bitcoin-0.10.0rc1/signature.tar.gz

And the final deterministic dmg is here:
https://bitcoincore.org/cfields/bitcoin-0.10.0rc1/bitcoin-0.10.0rc1-osx-signed.dmg

Needs backport to 0.10.