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.
I went with dataDir, I really think in the near future this should be obtained from a common place.