problems with database authentication

[abelsoares]

I try to use this script to backup a specific database on mongohq with no sucess
I have set a user on the database i want to backup and it's running mongo 2.4.4.

I set up DBAUTHDB and then find that the problem was with option --authenticationDatabase, which just didn´t work as expected.

It however works fine if i replace --authenticationDatabase with --db

I don´t know if this is a issue with the script or with my configurations, since i don't see no reason why --authenticationDatabase doesn´t work for my case.

i edited the script to fit this problem, adding new field DBNAME and pass --db if field is set otherwise use --authenticationDatabase if DBAUTHDB is set.

if open to pull request just say something.
Nice script by the way ;)