Bug fixes and UI updates:
* server.R now gets xlim from UI values, rather than from the data.frame. The latter resulted in an error if the range of the data.frame didn't match the range of the viewport.
* Bug fix: xlim now gets updated with isolate clause. Previously the xlimits were not updating correctly after UI changes. This lack of update would be noticeable after the addition of an annotation track.
* UI cleanup: Plot title depicts smaller ranges (< 1Mb)
* Plot legend now indicates number of samples in a group when samples are grouped
* Header panel reduced.
    - The section with active dataset is now part of the plot panel.
    - The welcome message is now not part of the lower header panel. It is at the top of the page and can be scrolled away. Maybe make it an alert that can be closed.
    This makes the fixed header section smaller.
* 'Refresh plot' button replaced by reload icon. There are now two icons, one at the fixed header bar and the other just above the plot.
* Instruction text made more succinct.
* Documentation: Added missing images for install instructions.