Add configuration options to build-ami/packer config

* `aws_access_key` and `aws_secret_key` now default to pulling from the
  AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY environment variables.
* `region` is now also settable and will default to AWS_DEFAULT_REGION if
  set in environment (follows convention used by aws cli tool)
* `source_ami` is now a required argument and defaults to the second
  aregument passed to `build-ami`
* `instance_type` is now a variable, still defaults to 'm1.large' but
  can be overriden with a `-var` switch to packer
Add new arg. for build-ami to Readme

Also some minor spelling fixes, cleaning up examples to reference node
instead of Rails, etc.
Remove hard-coded region from deploy script

The aws cli will use the AWS_DEFAULT_REGION environment variable or a
config file in ~/.aws/config if they're present
Merge pull request #1 from prognostikos/generalize-config

Generalize config