MDLSITE-2009 Various 'major' fixes and improvements

- Add support for changing version comments on release.
- Default for "next monday" and observe the --date option.
- Fix some problems with the newly created branch on major.
- Reorganize comments and notes.
MDLSITE-2009 Add "back-to-dev" build type.

To be executed once major releases have been packaged. It moves
master back to "dev" status.
Add 26_STABLE support to scripts.
Merge pull request #14 from stronk7/major_changes

MDLSITE-2009 Various 'major' fixes and improvements
Merge pull request #15 from stronk7/back-to-dev_changes

Back to dev changes
Merge pull request #16 from stronk7/add_26_stable_support

Add 26 stable support
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