Adjust header formatting

There are too many horizontal lines in the documentation, which makes knowing which things are headings (h2) and which are sub-headings (h3) almost impossible. 

I've tidied up the weights a little to make it more obvious and trimmed some excess whitespace. Ultimately I think a typographer's eye on the single choice of font-face (perhaps a serif for headings?) would be worthwhile.

_sass/_base.sass

@@ -712,7 +712,6 @@ dd
 		font-weight: 500
 		margin-bottom: 30px
 		padding-bottom: 10px
-		border-bottom: 1px solid #cccccc
 
 		// Make sure anchor links aren't hidden by the header
 		&:before
@@ -722,6 +721,9 @@ dd
 			height: $header-clearance
 			visibility: hidden
 
+	h1,h2
+		border-bottom: 1px solid #cccccc
+		
 	h1
 		font-size: 32px
 		padding-right: 60px
@@ -731,9 +733,12 @@ dd
 
 	h3
 		font-size: 24px
+		font-weight: 300
+		margin-bottom: 5px
 
 	h4
 		font-size: 20px
+		margin-bottom: 0px
 
 	h5, h6
 		font-size: 16px