Fix for issue #332

Now Opens shadow bucket with full server URL including user credentials if present, this allows password protected buckets to be opened.

A user must define a URL for the shadow bucket that includes credentials, e.g. in the sgconfig.json the following snippet creates a sync_gateway DB that shadows a password protected CouchbaseServer bucket. 

Note: The credentials username must match the shadow bucket name.

```
"databases": {
    "db": {
        "server":"http://localhost:8091",
        "bucket":"db",
        "shadow": {
            "server": "http://shadow:passw0rd@localhost:8091",
            "bucket": "shadow"
    },
```