change "Log In" to "Sign In"

because
http://www.designcult.org/2011/08/why-do-we-call-in-logging-in.html

spec/phantom_js/smoke_test.js

@@ -96,7 +96,7 @@ page.runTests = function(){
   });
 
   test("expect a log in button", function(){
-    return $('.current-username .btn').text() === 'Log In';
+    return $('.current-username .btn').text() === 'Sign In';
   });
 
   navigate("navigate to first topic", function(){