Make text operators <mi>s

Summary: According to the MathML spec [about
identifiers](http://www.w3.org/TR/MathML3/chapter3.html#id.3.2.3.3),
text function names should be <mi>s. This fixes that.

Fixes #202

Test plan:
 - `make test`

Auditors: alpert

test/katex-spec.js

@@ -1214,5 +1214,8 @@ describe("A MathML builder", function() {
 
         var text = getMathML("\\text{a}").children[0].children[0];
         expect(text.children[0].type).toEqual("mtext");
+
+        var textop = getMathML("\\sin").children[0].children[0];
+        expect(textop.children[0].type).toEqual("mi");
     });
 });