Updates to handle CMS/Blog theme development a bit better, as well as tighter integration to the @highlight and @accent color variables for quicker use of those colors to links, etc.

Added app/_/img/theme directory for segretating theme-based images into
their own location

Updated build/less/app.less for referencing global LESS variable
@img-url to reference the new img/theme path

Updated build/less/globals/colors.less to include @base, @highlight and
@accent variables

Added new parameters to the "a" reference within the
build/less/globals/defaults.less for handling hover/focus by default

Added hr parameter to the build/less/globals/defaults.less file for
more control over the hr tag's style