Datalog (#233)

* support for custom csv separator

* simple datalog frameowrk

* api strings

* hide setfile

* storage fixes

* log seconds, not milliseconds

pxtarget.json

@@ -16,8 +16,9 @@
         "libs/infrared-sensor",
         "libs/gyro-sensor",
         "libs/chassis",
-        "libs/storage",
         "libs/ev3",
+        "libs/storage",
+        "libs/datalog",
         "libs/tests",
         "libs/behaviors"
     ],