Fixed command not found error during tests on windows
Factor out a newBackupID function.
Add an alternate ID generator.
Add tests.
Merge pull request #658 from bogdanteleaga/uniter

Fixed command not found error during tests on windows

Added missing cmdSuffix in tests. Previously it was used only in the actual code. This fixes quite a number of tests on Windows.
Remove the old implementation.
Merge pull request #750 from ericsnowcurrently/backups-human-readable-ID

Make backup IDs more human-readable.

Currently we just use the current UUID.  This patch changes it to "<timestamp>.<env ID>".  A more human-readable ID will help juju users.