Enable Satis build script + add support for folder events

To be able to solve composer/satis#153, two changes are done:

### Split ArchiveManager->archive()
To be able to execute build script before archive generation, this function is refactored into two new public functions internally, while retaining same signature and behavior. 

### Add FolderEventInterface for events that centers around a folder
The folder is accessible on event from PHP scripts, and is passed on to ProcessExecutor for command line scripts to be in right current working directory, this could also be used by Package events in general as a followup.