The Great Rewrite (fixes #36, #61, #94, #101)

Rewrite of the file model and pulling mechanism. Needs lots of cleanup
and bugfixes, now...

files/debug.go

@@ -0,0 +1,12 @@
+package files
+
+import (
+	"log"
+	"os"
+	"strings"
+)
+
+var (
+	dlog  = log.New(os.Stderr, "files: ", log.Lmicroseconds|log.Lshortfile)
+	debug = strings.Contains(os.Getenv("STTRACE"), "files")
+)