In storage.acl.ACL.Entity constructor, check/verify types of input

From TODO:

``` python
      # TODO: Add validation of types.
```

See [`storage.acl.ACL.Entity`](https://github.com/GoogleCloudPlatform/gcloud-python/blob/1f2aaf3606802f4f02eeb30e22e1082ef7f4594e/gcloud/storage/acl.py#L104) for relevant code and #146 for some context.

`git blame` output

```
b7f1b774 (JJ Geewax   2014-01-29 18:02:29 -0500 104)       # TODO: Add validation of types.
```
