Add Stream API for Blobs

PHP offers a stream API that allows operations on services to look like low-level php filesystem operations.

This pull request adds support for this stream API for Windows Azure Blob Storage. The code was ported from the old SDK into the [AzureDistributionBundle](https://github.com/beberlei/AzureDistributionBundle) and from there now to this SDK.

This functionality is highly interesting for the SDK, because it has a wide range of use-cases with existing libraries that operate on PHP filesystem streams.

Example:

        $fileName = 'azure://my_container/test.txt';

        $blobClient = $this->createBlobClient();

        $fh = fopen($fileName, 'w');
        fwrite($fh, "Hello world!");
        $pos = ftell($fh);
        fseek($fh, 0);
        fwrite($fh, "Hello world!");
        fclose($fh);

        $result = file_get_contents($fileName);

        $this->assertEquals('Hello world!', $result);
        $this->assertEquals(12, $pos);

The Pull Request comes with 17 functional tests that are passing.