Support standard AWS credential environment variable names

Variable names used are defined here:

http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/SettingUp_CommandLine.html#set-aws-credentials
Added MongoDB support

manually merged Eric Lubow's changes from here:
https://github.com/elubow/ec2-consistent-snapshot
Updated the readme
Make changes to  kll/ec2-consistent-snapshot requested by ehammond prior to
merging mongodb features.
Remove unwanted AWS credentials option from README.
Merge pull request #1 from jabowery/master

MongoDB Merge Request
Merge pull request #9 from kll/master

Add MongoDB support, thanks to Kevin Lewis, Eric Lubow.