change retina-bases SASS variables

change values & names of the exiting media queries.  Add a new variable called '$mobile-non-retina'.
update 'grunt/cssSource/_retinaMediaQuery.scss' (see msg)

change SASS variable name from '$desktop-tablet' to '$desktop-tablet-non-retina'. edit a comment.
add retina version of bg image to the site
push dev & production copies of styles.css to GitHub
major grunt/cssSource/_retinaMediaQuery.scss updates (see msg)

redo all retina-based media queries, then browser/device test. place them in order of mobile-retina, mobile-non-retina, desktop-tablet-retina and desktop-tablet-non-retina.
push dev & production copies of styles.css to GitHub
continue clening up 'grunt/cssSource/_retinaMediaQuery.scss' (see msg)

add background image reference to all for retina-based media queries. start cleaning up comments.
push dev & production copies of styles.css to GitHub
more 'grunt/cssSource/_retinaMediaQuery.scss' updates (see msg)

mobile retina view now refers to all images. continue to clean up comments.
clean up 'grunt/cssSource/_retinaMediaQuery.scss' (see msg)

place 'mobile-retina' media query at the bottom or it gets over-ridden by 'mobile-non-retina'. remove duplicate '.site-title' class.
push dev & production copies of styles.css to GitHub
perform some final tasks for cleaning up retina-based media queries (see msg)

finish comments and checking in close-to-finished code before doing some testing, which can potentially damage things.
push dev & production copies of styles.css to GitHub
final tweaks before merge to the master branch (see msg)

Neaten up comments and do some small browser/device testing before merging the 'new-retina-media-queries' branch with the master branch.