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/blocksprj/main.ts

@@ -1,16 +1 @@
-basic.forever(() => {
-    basic.showLeds(`
-        . # . # .
-        # . # . #
-        # . . . #
-        . # . # .
-        . . # . .
-        `)
-    basic.showLeds(`
-        . . . . .
-        . . . . .
-        . . . . .
-        . . . . .
-        . . . . .
-        `)
-})
+