Godoc fixes (#898)

Add some godoc comments for the api/agent package and some of its
subpackages.

api/agent/drivers/doc.go

@@ -0,0 +1,14 @@
+// Package drivers is intended as a general purpose container abstraction
+// library. It abstracts across the differences between different container
+// runtimes (e.g. Docker, Rkt, etc.) and provides utlities and data types that
+// are common across all runtimes.
+//
+// Docker Driver
+//
+// The docker driver runs functions as Docker containers.
+//
+// Mock Driver
+//
+// The mock driver pretends to run functions but doesn't actually run them. This
+// is for testing only.
+package drivers