adds option to search for particular plugin directories
use ASUtils to locate plugin migration directories
use ASUtils to locate plugins when initializing
add plugin assets to rails pipeline in production as well as development
Merge pull request #11 from lcdhoffman/plugin-migrations-patch

Plugin migrations patch
EAD export: change DAO href mapping

Use the first `file_version.file_uri` if available; otherwise
use the `digital_object.digital_object_id`
Merge pull request #12 from anarchivist/dao-uri-mapping

EAD dao mapping changes
Remove unecessary require statement [ci skip]