Fix inconsistent and incorrect quotes

Quotes simply aren't needed in `url()` CSS, and these were inconsistently `'`, `"` and non-existent.

No need for double-quoted string for the meta generator markup, and single-quoted PHP string allows for double-quoted HTML attributes, as is consistent with 99% of the response markup.
Fix code alignment

Mid-line alignments should be with spaces, not tabs.
Fix yoda conditionals

As per WP coding standards, when comparing a string against a variable, the string should be on the left. If not, then it's easy to introduce a logic error by accidentally doing an assignment (`=`) instead of a comparison (`==`) which can be tricky to track down.
Fix array trailing comma inconsistency

For arrays with more than one item, a trailing comma after the last item is useful, since new items can be added easily, and items can be re-ordered without introducing syntax errors.

Empty arrays don't need whitespace inside them.
Refactor duplicate path strings

Introduce local variables to refactor out duplication of path strings. These values could be further refactored out into their own functions for further re-use / less duplication.
Refactor edd_register_styles()

Lots of duplication in there that can be refactored out to make the code a little more self-documenting. Ideally, the task of finding which style sheet should be used (line 100 to 117 on this commit) would be moved to its own function, leaving the registering only in the current function.
Add local variables for internal style sheet image references
Merge pull request #1135 from GaryJones/scripts

includes/scripts.php