Change default token to empty string instead of undefined

fixes #329 

When ACL is turned on, the UI sends requests with `token=undefined` request parameter. This cause the 403 error.

By defaulting the token to empty string fixes this issue. 

## Alternative solution

Quote from the [ACL documentation](http://www.consul.io/docs/internals/acl.html)
> This is handled by the special "anonymous" token. Anytime there is no token provided, the rules defined by that token are automatically applied. 

So maybe `token=anonymous` would be more explicit. What do you think guys?