LIBCLOUD-80: Windows Azure Storage driver

Implement storage driver for Windows Azure Blob Storage.

http://msdn.microsoft.com/en-us/library/windowsazure/dd135733.aspx

The driver supports the following
* Azure Block Blobs support (default)
* Azure Page Blobs (ex_page_type option)
* Support for leasing blobs while being updated (ex_use_lease option)