dominant-baseline:central for vertical centering

the latest of many attempts to get this right
previously we had dominant-baseline:text-after-edge and dy 0.5em
but that didn't seem tall enough

ref f0f6409 / c574c96 for last attempt

i don't understand why i upvoted
https://stackoverflow.com/questions/12250403/vertical-alignment-of-text-element-in-svg
but still tried to offset the bottom of the text instead of using `central`

we'll see..... fixes #96 ?

Author: gordonwoodhull

dc.graph.css

@@ -13,7 +13,7 @@
 .dc-graph text.node-label {
     font: bold 14px verdana, sans-serif;
     text-anchor: middle;
-    dominant-baseline: text-after-edge;
+    dominant-baseline: central;
     pointer-events: none;
 }
 

src/node_contents.js

@@ -13,7 +13,7 @@ dc_graph.text_contents = function() {
                 else if(typeof lines === 'string')
                     lines = [lines];
                 var lineHeight = _contents.parent().nodeLineHeight();
-                var first = 1 - ((lines.length - 1) * lineHeight + 1)/2;
+                var first = 0.5 - ((lines.length - 1) * lineHeight + 1)/2;
                 return lines.map(function(line, i) { return {node: n, line: line, yofs: (i==0 ? first : lineHeight) + 'em'}; });
             });
             tspan.enter().append('tspan');