Improve Features support

In its current implementation, each rule is exported without reference to its "weight" field. As this module's functionality hinges upon the order of rules in the chain, this oversight makes it impossible to revert a site back to a known-good state after misconfiguration.

Also, the Features UI displays an md5 hash of each rule, which makes it difficult to comprehend which rules appear in different modules. It may be preferable to use a machine-readable version of the "string" field already being generated for export as the key for import/export.