[WIP] Add base url support

Following #51 , this PR adds base url support to HTTP adapters.
I don't think they are *absolutely* needed, but the make the usage more comfortable. Without these, we would have to do it like this

```php
$messageFactory = new MessageFactory();
$messageFactory->setBaseUrl('http://example.com');

$configuration = new Configuration($messageFactory);
$http = new GuzzleHttpHttpAdapter($configuration);
// ...
$hasBaseUrl = $http->getConfiguration()->getMessageFactory()->hasBaseUrl();
```

which looks not as comfortable as

```php
$configuration = new Configuration();
$configuration->setBaseUrl('http://example.com');
$http = new GuzzleHttpHttpAdapter($configuration);

// ...

$hasBaseUrl = $http->getConfiguration()->getBaseUrl();
```

Besides, somehow it really feels like a *configuration* thing to me, which I (personally) would expect to find in a `Configuration` object, rather than to have to configure a `MessageFactory` and then configure a `Configuration` with it, at least from and "end user" (developer) point of view.