Display BigWig coverage along with BAM track and various fixes.

- Displays BigWig coverage track along with BAM track if coverage file exists for given BAM file,
- Window height can be set to fixed amount,
- Fixed exposing of public functions,
- Fixed async bug
If jbrowse directive will ever appear twice on the same page, we must also get rid of all id attributes `id="..."` and id selectors `$('#...')`.

Most can usually be replaced with a class and selected by `$element.find('.class')` (only searches within directive's root)