Drift-compensated angle in gyro (#931)

* compute angle based on undrifted rate

* add is calibrating function

* fix integrator

* added example

* docs

* poll faster

libs/core/pxt.json

@@ -25,7 +25,8 @@
         "dal.d.ts",
         "icons.jres",
         "ns.ts",
-        "platform.h"
+        "platform.h",
+        "integrator.ts"
     ],
     "testFiles": [
         "test.ts"