Accessibility improvement

Because they don't have a valid href, <a> tags are neither keyboard focusable (tab after the textbox) nor presented as link. 
Since they have the class btn and look like buttons, I decided to apply the role button instead of link.

Server/views/httpproxy.jade

@@ -22,10 +22,10 @@ html(lang='en')
         input#url(type='text', placeholder='enter website url')
       div.button
         div.spacebutton
-          a#inject.btn.btn-default.left Inspect with VORLON.JS
-          a#openSite.btn.btn-default.left Open website only
-          a#openDashboard.btn.btn-default.left Open dashboard only
-          a#error.message.left.error.hide URL is not valid
+          a#inject.btn.btn-default.left(role='button', tabindex='0') Inspect with VORLON.JS
+          a#openSite.btn.btn-default.left(role='button', tabindex='0') Open website only
+          a#openDashboard.btn.btn-default.left(role='button', tabindex='0') Open dashboard only
+          a#error.message.left.error.hide(role='button', tabindex='0')URL is not valid
 
  script(type='text/javascript').
  	$(document).ready(function() {	
@@ -109,4 +109,4 @@ html(lang='en')
      urlinput.addEventListener("blur", function (e) {
        checkUrl();
      });
- 	});
+ 	});