Revamp icons

Add support for highlighted icons
Add non-retina versions for all icons
Add new icons for tag, fetch, pull, push, detail and tree
Add icons for collapsed and expanded folder items
Refine colors for file icons
Rename Images.pxm to Icons.pxm
Pixel-align html action icons
Pixel align hires icons and buttons
Pixel align icons and buttons
Correct pixel aligned buttons
Merge branch 'refs/heads/master' into issues/190
Add pull request #190 to the change log

Also, put pull request #203 under the correct version
Merge branch 'issues/190'