MDLSITE-2009 prerelease: several minor improvements

* Moved allbranches var to global space so that others can use it.
  Converted uses to the new global as appropriate.
* Converted echo usage within reset_repo and show_changes to use output
  allowing their use within scripts in conjunction with --quiet.
* Added branch existence checking and responsible output to reset_repo
  and show_changes so that errors are avoided when expected branches
  are missing.
* Minor help formatting improvements.
MDLSITE-2009 tidied up the README.md file
Merge pull request #17 from samhemelryk/MDLSITE-2009

MDLSITE-2009 minor improvements to prerelease.sh and README.md