[merge addon] Add highlightDifferences option and method

Provides new option (highlightDifferences) and method highlightDifferences(bool)
to the MergeView plugin.

If true (or undefined) the original behaviour is maintained, which is to show
the differences (line highlighting and gutter graphics), if false then
the changes will not be shown.

Calling the method causes the visual change to occur 'immediately'

Signed-off-by: ciaranj <ciaranj@gmail.com>
[merge addon] Clean up showDifferences option

Issue #1765
Add "gutterContextMenu" event to capture context menu events in gutters
Clean up gutterContextMenu code

Issue #1766
[project page] Set border: 0 for logo

Woah, IE still defaults to a border around images in links.
Fix bug introduced by previous contextmenuInGutter patch

Issue #1766
Use zero-width space in context-menu select-all hack

To prevent the mouse click from landing outside of the selected
text, which broke copy in IE.

Closes #1764