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"

libs/core/control.ts

@@ -31,7 +31,7 @@ namespace control {
     /**
      * If the condition is false, display msg on serial console, and panic with code 098.
      */
-    export function assert(condition: boolean, msg ?: string) {
+    export function assert(condition: boolean, msg?: string) {
         if (!condition) {
             console.log("ASSERTION FAILED")
             if (msg != null) {