Add :min_adjustment_step option for AutoScaling policies

Not sure about which option to choose between `:min_adjustment_step` and `:min_adjustment_magnitude`.

According to the [API Reference](http://docs.aws.amazon.com/AutoScaling/latest/APIReference/API_PutScalingPolicy.html), the correct option should be `:min_adjustment_step`.

That's my first pull request, I'm a bit lost about tests. If I understand well, I should add Cucumber tests (via the `features` directory) to cover theses changes.

Currently when I execute `rake features` I get the following error :

    # rake features
    (in /mnt/workspace/aws-sdk-ruby)
    undefined method `empty?' for nil:NilClass (NoMethodError)
    /mnt/workspace/aws-sdk-ruby/lib/aws/core.rb:461:in `config'
    /mnt/workspace/aws-sdk-ruby/features/support/common.rb:64:in `AfterConfiguration'
    rake aborted!
    Command failed with status (256): cucumber features --format pretty --tags ~@wip
    /mnt/workspace/aws-sdk-ruby/tasks/cucumber.rake:18:in `block in <top (required)>'
    /usr/lib/ruby/1.9.1/rake.rb:634:in `call'
    /usr/lib/ruby/1.9.1/rake.rb:634:in `block in execute'
    /usr/lib/ruby/1.9.1/rake.rb:629:in `each'
    /usr/lib/ruby/1.9.1/rake.rb:629:in `execute'
    /usr/lib/ruby/1.9.1/rake.rb:595:in `block in invoke_with_call_chain'
    /usr/lib/ruby/1.9.1/monitor.rb:201:in `mon_synchronize'
    /usr/lib/ruby/1.9.1/rake.rb:588:in `invoke_with_call_chain'
    /usr/lib/ruby/1.9.1/rake.rb:581:in `invoke'
    /usr/lib/ruby/1.9.1/rake.rb:2041:in `invoke_task'
    /usr/lib/ruby/1.9.1/rake.rb:2019:in `block (2 levels) in top_level'
    /usr/lib/ruby/1.9.1/rake.rb:2019:in `each'
    /usr/lib/ruby/1.9.1/rake.rb:2019:in `block in top_level'
    /usr/lib/ruby/1.9.1/rake.rb:2058:in `standard_exception_handling'
    /usr/lib/ruby/1.9.1/rake.rb:2013:in `top_level'
    /usr/lib/ruby/1.9.1/rake.rb:1992:in `run'
    /usr/bin/rake:31:in `<main>'

Any help welcome. Thanks.