Add Azure Storage repository

elasticsearch 1.0 will provide a new feature named `Snapshot & Restore`.

We want to add support for [Azure Storage](http://www.windowsazure.com/en-us/documentation/services/storage/).

To enable Azure repositories, you have first to set your azure storage settings:

```yaml
    cloud:
        azure:
            storage_account: your_azure_storage_account
            storage_key: your_azure_storage_key
```

The Azure repository supports following settings:

* `container`: Container name. Defaults to `elasticsearch-snapshots`
* `base_path`: Specifies the path within container to repository data. Defaults to empty (root directory).
* `concurrent_streams`: Throttles the number of streams (per node) preforming snapshot operation. Defaults to `5`.
* `chunk_size`: Big files can be broken down into chunks during snapshotting if needed. The chunk size can be specified
in bytes or by using size value notation, i.e. `1g`, `10m`, `5k`. Defaults to `64m` (64m max)
* `compress`: When set to `true` metadata files are stored in compressed format. This setting doesn't affect index
files that are already compressed by default. Defaults to `false`.

Some examples, using scripts:

```sh
$ curl -XPUT 'http://localhost:9200/_snapshot/my_backup1' -d '{
    "type": "azure"
}'

$ curl -XPUT 'http://localhost:9200/_snapshot/my_backup2' -d '{
    "type": "azure",
    "settings": {
        "container": "backup_container",
        "base_path": "backups",
        "concurrent_streams": 2,
        "chunk_size": "32m",
        "compress": true
    }
}'
```

Example using Java:

```java
client.admin().cluster().preparePutRepository("my_backup3")
        .setType("azure").setSettings(ImmutableSettings.settingsBuilder()
                .put(AzureStorageService.Fields.CONTAINER, "backup_container")
                .put(AzureStorageService.Fields.CHUNK_SIZE, new ByteSizeValue(32, ByteSizeUnit.MB))
        ).get();
```

Closes #2.