Make blog title and description more legible on light backgrounds

Adds a light shadow to blog title and description to give slightly more contrast between text and background when a cover image is used with a predominently light colour. This is not a complete fix, because heavy shadows do not lend to the aesthetic of Casper - however it does go some way to making a small improvement to legibility in most cases.

Related: TryGhost/Ghost#1768

assets/css/screen.css

@@ -397,6 +397,7 @@ margin on the iframe, cause it breaks stuff. */
     letter-spacing: -1px;
     font-weight: bold;
     font-family: 'Open Sans', sans-serif;
+    text-shadow: 0 1px 6px rgba(0,0,0,0.1);
 }
 
 .blog-description {
@@ -406,6 +407,7 @@ margin on the iframe, cause it breaks stuff. */
     font-weight: 300;
     font-family: 'Noto Serif', serif;
     letter-spacing: 0;
+    text-shadow: 0 1px 3px rgba(0,0,0,0.15);
 }
 
 /* Every post, on every page, gets this style on its <article> tag */