CRM-11658 - "releaser" for git - Changes from old "build-tarballs":

CRM-11658 - "releaser" for git - Changes from old "build-tarballs":
 * Fetch code with "gitify"
 * Make coordinated tags and push them (instead of using post-commit hook on single repo)
 * Extract hard-coded references to sf.net
 * Split into smaller functions (which are more explanatory and easier to toggle for testing)
 * Document configuration file format
 * Rename executable variables (e.g. $php => $cmd_php)
 * Use "set -ex" to catch errors