busybird.js, timeline.js: move StatusContainer from busybird.js to timeline.js.
xt/js/status_container.html: add tests for set/getThresholdLevel() methods.
xt/js/status_container: temporarily set bb.ANIMATE_STATUS_MAX_NUM, so that we can test both animation and non-animation cases.
xt/js/status_container: create FakeStatusServer for testing.
xt/js/status_container: add tests for loadStatuses().
timeline.js: StatusContainer: implement loadStatuses(). Pass the tests.
timeline.js: property names of interface objects are now in camelCase instead of snake_case. camelCase names are more common among JavaScript objects.
"use strict" for all JavaScript codes.
minor quality fixes for JS codes.