Add documents (merges foodcoops/foodsoft#353)

plugins/documents/db/migrate/20160220000000_create_documents.rb

@@ -0,0 +1,15 @@
+class CreateDocuments < ActiveRecord::Migration
+  def self.up
+    create_table :documents do |t|
+      t.string :name
+      t.string :mime
+      t.binary :data, limit: 16.megabyte
+      t.integer :created_by_user_id
+      t.timestamps
+    end
+  end
+
+  def self.down
+    drop_table :documents
+  end
+end