CouchbaseCache.get(<key>, <valueLoader>) more resilient

Hi theres,

I've been testing this library over the past days and I have a concern/doubt about how the method 'CouchbaseCache.get(, )' behaves when an exception in Bucket.get is thrown.

As I could notice none of the possible runtime exceptions that are throwed are handled in, what makes my applications fails when Couchbase has a temporarily failure. Could this method catch all these exceptions and just call the valueLoader?

Something Like:
```@Override
public T get(final Object key, final Callable valueLoader) {
try {
final String documentId = getDocumentId(key.toString());
SerializableDocument doc = client.get(documentId, SerializableDocument.class);
if (doc == null && valueLoader != null) {
synchronized (client) {
doc = client.get(documentId, SerializableDocument.class);
if (doc == null) {
T value = callValueLoader(key, valueLoader);
put(key, value);
return value;
}
}
}

if (doc != null) {
return (T) doc.content();
}
} catch(Exception ex) {
//log error
return callValueLoader(key, valueLoader);
}
return null;
}

private T callValueLoader(final Object key, final Callable valueLoader) {
try {
return valueLoader.call();
} catch (ValueRetrievalException ex) {
throw ex;
} catch (Exception e) {
throw new ValueRetrievalException(key, valueLoader, e);
}
}
```

Let me know whether this makes sense to you, so I can create a pull request with tests and a better code than this one on the example.

Thanks,