Add fields for proxy credentials & add windows 7& 8 support

- proxyUser and proxyPassword must both be specified for credential proxy user.
- proxyPassword must be a securestring unless isProxySecure is manually set false
- Add Windows 7 and 8 support to metadata.json
- Change Readme to reflect Windows 7 and 8 support
- Update readme to reflect new variables
- Add erb validation tests and fix previous erb tests