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.
Slight tweaks to the arrangement of the readme.
If a project tag is being used, include it in the output from ec2:status.
Updated changelog for v1.4.8.
Fetch and set the real revision (eg/ Git ref) before beginning the rolling deployment, so any changes made to SCM during deployment will not be inadvertently included.
Update changelog for v1.4.9.