MVP Gulp introduction (#334)

* Install gulp as dev dependency

* Placeholder gulpfile

* Replace rollup invocations with gulp

* Consolidate duplicated code

* Big cleanup of gulpfile

* Without cache test

* Revert "Without cache test"

This reverts commit 0e3881d8d7.

demo_data/demo_data.js

@@ -0,0 +1,5 @@
+import bootstrap from './bootstrap_data';
+import logbook from './logbook_data';
+import stateHistory from './state_history_data';
+
+window.hassDemoData = { bootstrap, logbook, stateHistory };