option to specify a script that runs after backups

This script could be used to sync backups to a central server, or just
to create a file to let external scripts know that backups are
(sucessfully) done and can be picked up.

The postscript option (defaults to false) and can be either a string or
an array of strings. These strings will be directly put into the
mysqlbackup.sh and as such can either be shell commands, or externally
managed files.
Fix failing backup spec tests

This partially reverts 7224f12c250a20fa24935a9026ada81269b7fce8
since after #416 the warnings are actually fixed at the source
Allow configuration of Gem source in Gemfile

This commit allows configuring of the Gem source via an environment
variable "GEM_SOURCE" with a default of 'https://rubygems.org'.
This follows the conventions of Puppet Labs Open Source Platform team.
Merge pull request #413 from igalic/backup-postscript

option to specify a script that runs after backups
Merge pull request #443 from igalic/fix-failing-backup-spec

Fix failing backup spec test
Merge pull request #444 from justinstoller/master_gem_source

Allow configuration of Gem source in Gemfile
(maint) Fix the specs due to changes to backup.
Merge pull request #446 from apenney/fix-specs

(maint) Fix the specs due to changes to backup.