Add session token support for use with STS
Add aws_session_token option to the README
Remove short option from aws_session_token (-T in use)
Add guard/rvm/rbenv files to gitignore
Add test for aws-session-token option
Use locate_config_value to find session token
Provide STS token example in README
Documentation updates for AWS STS Token
Merge pull request #211 from opscode/adamed/knife-464

KNIFE-464: Support EC2 STS