On start support (#324)

* annotating APIs to support onstart

* missed file

* updated templates

* adding new lines in main.ts

* fixing docs

* removing onstart

* updated docs

* updated various docs

* more docs

* adding upgrade policies for blocks

* updated pxt reference

* placing on start under "basic"

package.json

@@ -34,6 +34,6 @@
     "semantic-ui-less": "^2.2.4"
   },
   "dependencies": {
-    "pxt-core": "0.6.7"
+    "pxt-core": "0.7.1"
   }
 }