Added bar that appears when user is zoomed at a level where individual bases are displayed.

This addition has been made by modifying the existing ruler2 by applying styles conditioned upon zoom level and also pixel density of screen. To be added: exact location above base bar; option to disable base bar.
Added location of single nuc above ruler when zoomed in.

Added a view listener to update the innerHTML of the locSingleBase attribute. In Browser.prototype.positionRuler() in cbrowser.js, the locSingleBase is made visible/hidden depending on zoom level. At present the locSingleBase location coincides with ruler2 and a future change will have it centered above ruler2.
Re-styled ruler2. ruler2 width now tracks scale. Attempting to tweak marriage of locSingleBase text and base under ruler2.
Offset for locSingleBase now accomodates for the ruler2 width.
Added genome location highlight checkbox to options table.
Merge branch 'singleBaseHighlight' of https://github.com/dlrice/dalliance into singleBaseHighlight
Integrated singleBaseHightlight option in browser.

Added listener to singleBaseHighlight button to update user selection.
Added conditions in updateRuler to toggle display of singleBaseHighlight
ruler/standard ruler. Added singleBaseHighlight entry in session.js.
Adding styles to ruler2 to ensure it is a grey line if the user deselects show single base highlighting
Merge pull request #111 from dlrice/singleBaseHighlight

Single base highlight and location display