Merge pull request #19 from arjenvanderende/master

Adds ability to overwrite existing SimpleDB rows
added support for ec2-describe-instances, ec2din for short
Using DefaultAWSCredentialsProviderChain when key and secret aren't provided
Merge pull request #20 from q42jaap/ec2din

added support for ec2-describe-instances, ec2din for short thanks to @q43jaap
Merge pull request #22 from q42jaap/awscreds

Using DefaultAWSCredentialsProviderChain when key and secret aren't provided