Storage has now it's own package.

Remove global `sessions` map and use configured storage.
Add a `types` package so both `pwd` and `storage` can access without
circular dependencies.
Now the session is prepared when requested and not on load.

pwd/types/client.go

@@ -0,0 +1,12 @@
+package types
+
+type Client struct {
+	Id       string
+	ViewPort ViewPort
+	Session  *Session
+}
+
+type ViewPort struct {
+	Rows uint
+	Cols uint
+}