Add ant target to delete only stale metadata from an org

Fixes #396
- Added new target, destroyStaleMetadata, which removes any metadata in the Cumulus package in the target org which is not in the repository being deployed

- Added new target, deployCIPackageOrg, which swaps destroyStaleMetadata in place of uninstallCumulus so it can run against the package org

- Fixes #435  With this change you can use a build.properties file in the root of the repo to provide credentials to ant without specifying the file via the command line.  If you specify a properties file via the command line which sets sf:username, the build.properties is skipped.