Add REVISION file and support current revision for all scms

This addresses #889 #923 and #935.  It's a much more robust replacement for #923.

- `current_revision` is supported in all Default SCM strategies
- Each release includes a REVISION file.
- `previous_revision` is set during deploy:starting, by reading from "#{previous_release_path}/REVISION" if it exists

What's missing:
- automated tests.  @leehambley, please let me know if any tests would be appropriate.
- The SVN default strategy's fetch_current_revision is not so graceful, so there may be room for improvement here.