Make all the lines the correct sizes

Fix the \frac and \overline lines so they actually render at the correct sizes
(i.e. don't get smaller when in script and scriptscript sizes). This also seems
to fix #33.

Test Plan:
 - Make sure new huxley screenshot look good.
 - Render
     \frac{1}{\Bigl(\sqrt{\phi \sqrt{5}}-\phi\Bigr) e^{\frac25 \pi}} =
     1+\frac{e^{-2\pi}} {1+\frac{e^{-4\pi}} {1+\frac{e^{-6\pi}}
     {1+\frac{e^{-8\pi}} {1+...} } } }
   at small font sizes like 30pt and smaller, see that it looks much better

Auditors: alpert

buildTree.js

@@ -279,11 +279,13 @@ var groupTypes = {
         var fontSizer = buildCommon.makeFontSizer(options,
             Math.max(numer.maxFontSize, denom.maxFontSize));
 
+        var line = makeSpan([options.style.reset(), Style.TEXT.cls(), "line"]);
+
         var numerrow = makeSpan(["mfracnum"], [fontSizer, numernumer]);
-        var mid = makeSpan(["mfracmid"], [fontSizer, makeSpan(["line"])]);
+        var mid = makeSpan(["mfracmid"], [fontSizer, line]);
         var denomrow = makeSpan(["mfracden"], [fontSizer, denomdenom]);
 
-        var theta = fontMetrics.metrics.defaultRuleThickness;
+        var theta = fontMetrics.metrics.defaultRuleThickness / options.style.sizeMultiplier;
 
         var u, v, phi;
         if (fstyle.size === Style.DISPLAY.size) {
@@ -498,10 +500,11 @@ var groupTypes = {
         var fontSizer = buildCommon.makeFontSizer(options, innerGroup.maxFontSize);
 
         // The theta variable in the TeXbook
-        var lineWidth = fontMetrics.metrics.defaultRuleThickness;
+        var lineWidth = fontMetrics.metrics.defaultRuleThickness /
+            options.style.sizeMultiplier;
 
         var line = makeSpan(
-            ["overline-line"], [fontSizer, makeSpan(["line"])]);
+            ["overline-line"], [fontSizer, makeSpan([options.style.reset(), Style.TEXT.cls(), "line"])]);
         var inner = makeSpan(["overline-inner"], [fontSizer, innerGroup]);
         var fixIE = makeSpan(
             ["fix-ie"], [fontSizer, new domTree.textNode("\u00a0")]);

static/katex.less

@@ -228,7 +228,7 @@ big parens
 
             &:after {
                 border-bottom-style: solid;
-                border-bottom-width: 0.05em;
+                border-bottom-width: 0.04em;
                 content: "";
                 display: block;
                 margin-top: -1px;