add(master_release): Add script for updating master's version.txt

This is intended to be called after branch_release and all official
builds from that new branch are complete. Then updating master's
version.txt will switch things to start using that new release as the
source for binary packages.

Complete documentation for this whole process coming soon. :)
fix(*_release): Calculate TODAYS_VERSION in common.sh
Merge pull request #16 from marineam/version

add(master_release): Add script for updating master's version.txt