Allow the use of IAM instance roles

Hi,

I'm using IAM instance roles, creating a new instance role for each instance I create using Cloudformation.  I don't have to specify keys, etc. when I run mongo_lock_and_snapshot.  Instead, I can just get credentials out of the metadata service.

Therefore, I moved some of the connection related code to a new class: EC2Helper.  I've tested it through a few scenarios (using root AWS credentials and IAM roles) and I've been able to do nearly everything the gem can.

Also, in this pull request are some simple spelling fixes and just a tad bit of love for @geemus's fog.

Thanks!