Add support for Azure storage as a publishing backend

This adds a new configuration setting: AzurePublishEndpoints, similar
to the existing S3PublishEndpoints and SwiftPublishEndpoints.

For each endpoint, the following has to be defined:
 - accountName
 - accountKey
 - container
 - prefix

Azure tests require the following environment variables to be set:
 - AZURE_STORAGE_ACCOUNT
 - AZURE_STORAGE_ACCESS_KEY

With either of these not set, Azure-specific tests are skipped.

azure/azure_test.go

@@ -0,0 +1,12 @@
+package azure
+
+import (
+	"testing"
+
+	. "gopkg.in/check.v1"
+)
+
+// Launch gocheck tests
+func Test(t *testing.T) {
+	TestingT(t)
+}