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/integrator.ts

@@ -0,0 +1,26 @@
+namespace control {
+    export class EulerIntegrator {
+        public value: number;
+        private t: number;
+        private v: number;
+
+        constructor() {
+            this.reset();
+        }
+
+        public integrate(derivative: number): void {
+            let now = control.millis();
+            let dt = (now -this.t) / 1000.0;
+            this.value += dt * (this.v + derivative) / 2;
+
+            this.t = now;
+            this.v = derivative;
+        }
+
+        public reset() {
+            this.value = 0;
+            this.v = 0;
+            this.t = control.millis();
+        }
+    }
+}