Various fixes (#548)

* adding running time micros

* support for system micro seconds

* restore display mode after plotting sprites

* bump pxt

docs/reference/input/running-time-micros.md

@@ -0,0 +1,17 @@
+# Running Time Micros
+
+Find how long it has been since the program started in micro-seconds.
+
+```sig
+input.runningTimeMicros();
+```
+
+## Returns
+
+* the [Number](/types/number) of microseconds since the program started.
+(One second is 1000000 microseconds.)
+
+## See also
+
+[show number](/reference/basic/show-number), [pause](/reference/basic/pause)
+

docs/reference/input/running-time.md

@@ -1,6 +1,6 @@
 # Running Time
 
-Find how long it has been since the program started.
+Find how long it has been since the program started in milli-seconds.
 
 ```sig
 input.runningTime();