Fix x-tags when used with AMD

When x-tags is used as an AMD module, it expects `xtag` to be in the global scope, but it won't be there if `define()` is present and AMD-compatible. `xtag` should simply always be in the global scope.

/cc @potch @spasovski
Merge pull request #28 from mattbasta/patch-1

Fix x-tags when used with AMD