modularize util.isAbsolute

by using a polyfill of the Node 0.12 `path.isAbsolute()` method

https://github.com/sindresorhus/path-is-absolute

lib/babel/util.js

@@ -69,13 +69,6 @@ exports.arrayify = function (val) {
   throw new TypeError("illegal type for arrayify");
 };
 
-exports.isAbsolute = function (loc) {
-  if (!loc) return false;
-  if (loc[0] === "/") return true; // unix
-  if (loc[1] === ":" && loc[2] === "\\") return true; // windows
-  return false;
-};
-
 var templateVisitor = {
   enter: function (node, parent, scope, nodes) {
     if (t.isExpressionStatement(node)) {