Account for top offset in hit test

Previously, HitTester.testItemMap() would not take into account
the offsetTop when testing for an event in the hit area. This is
used in peek mode to determine if there is a mouse move or click in a
particular bounding box on the page. The bounding box coordinates are
expected to be determined relative to the page, and the offsetTop
should be added to determine where this is.

How To +10/QA
-------------

```bash
# skip the following steps if you've already initialized the project
$ git clone git@github.com:WebFilings/wf-uicomponents.git
$ cd wf-js-uicomponents

# do not skip these!
$ git fetch && git checkout account_for_top_offset_in_hit_test
$ ./init.sh
$ grunt qa
```

All unit tests should pass.