Allow user to specify the tag used for Project matching

Our ops team uses a tag of "app" instead of "Project" to set the project for an instance.

- add aws_project_tag option and default to "Project"
- switch project_instances to use the aws_project_tag setting
- update the readme with the new setting
bump version number
Merge pull request #29 from skisulli/aws_project_tag

Allow user to specify the tag used for Project matching.