Add package metadata to slug file format

We're starting to use the SH package format for a number of different project types. As part of this, we're adding some metadata to the packages so that we can easily determine how to deploy them. For example, a Rails project will want to roll unicorns, but a package of JavaScript assets would be installed differently. For other users, this is a no-op, with the exception of seeing a new `__METADATA__` line in the file, just prior to the `__ARCHIVE__` line.
@jordansissel