Re wrote backup in go

Previously backup was a cmd plugin writen in bash.
Backup was implemented as a go function, still not in use.
Tests where added since the previous implementation had none.
backup package includes:
 * BackUp function, which performs the actual back up.
 * tarFiles which creates a tar or tar.gz/tgz depending
on the output file name given.
 * runCommand which is a convenience function to simplify testing
since the back up process requires calling external executable
mongoexport.
This won't work - you're passing a shell-style command line to this argument.

I think you want something like:

func _runCommand(cmd string, args ...string) (string, error) {
    c := exec.Command(cmd, args...)
    etc
}

and then change the caller to pass the arguments as individual
strings.

BTW "dumpCommand" isn't a great name here - it could be any
kind of command.