add mongodb backup script

db.isMaster() returns something like this:

> connecting to: stage
> {
> 	"setName" : "rs0",
> 	"setVersion" : 1,
> 	"ismaster" : false,
> 	"secondary" : true,
> 	"hosts" : [
> 		"stage-mongo-0.mckinsey.edx.org:27017",
> 		"stage-mongo-2.mckinsey.edx.org:27017",
> 		"stage-mongo-1.mckinsey.edx.org:27017"
> 	],
> 	"primary" : "stage-mongo-1.mckinsey.edx.org:27017",
> 	"me" : "stage-mongo-0.mckinsey.edx.org:27017",
> 	"maxBsonObjectSize" : 16777216,
> 	"maxMessageSizeBytes" : 48000000,
> 	"maxWriteBatchSize" : 1000,
> 	"localTime" : ISODate("2014-08-15T16:02:23.686Z"),
> 	"maxWireVersion" : 2,
> 	"minWireVersion" : 0,
> 	"ok" : 1
> }