version.sh: ditch SVN version numbers completely and use git describe

Side note: As a cosmetical change, if no git is present, the hash won't
be added to the full version string anymore as useless " (00000000)".
version.sh: refactor to improve messages when no git available or not a git repo
version.sh / mpc-hc.exe.manifest: generate full version number

Get rid of the redundant definition of the fixed version number
in mpc-hc.exe.manifest.conf.

Instead, use the values from version.h and write the whole version
to the manifest file.

version.sh is now the only place where version numbers need to
be changed.
version.sh: add two more echo commands to print some useful information

- echo MPC_VERSION_MAJOR.MPC_VERSION_MINOR.MPC_VERSION_PATCH
  gathered from version.h
- echo full git-describe output
Update docs/Release.txt for the version changes.
version.h: Don't mention the revision in stable releases.

It will always be 0 in that case.
Rename Beta to Nightly.