Add :previous_revision early in the Cap run

@betesh: I elected to leave the duplication around `last_release` for another day. That's dealing with timestamps/directory names. What we care about for `:previous_revision` is SHAs.

I have added this early into the Capistrano run with some basic checking to ensure that the file and directory already exit, if not the variable won't be set.

This should lay the groundwork for `cap deploy:pending` and certainly provide the features required for change logs