Add Azure Blob Storage as backend

Environment variables:
AZURE_ACCOUNT_NAME=storage-account-name
AZURE_ACCOUNT_KEY=storage-account-key

Environment variables for test:
RESTIC_TEST_AZURE_ACCOUNT_NAME=storage-account-name
RESTIC_TEST_AZURE_ACCOUNT_KEY=storage-account-key
RESTIC_TEST_AZURE_REPOSITORY=azure:restic-test-container

Init repository:
$ restic -r azure:container-name:/prefix/dir init

internal/backend/azure/config.go

@@ -0,0 +1,57 @@
+package azure
+
+import (
+	"errors"
+	"path"
+	"strings"
+
+	"github.com/restic/restic/internal/options"
+)
+
+// Config contains all configuration necessary to connect to an azure compatible
+// server.
+type Config struct {
+	AccountName string
+	AccountKey  string
+	Container   string
+	Prefix      string
+
+	Connections uint `option:"connections" help:"set a limit for the number of concurrent connections (default: 20)"`
+}
+
+// NewConfig returns a new Config with the default values filled in.
+func NewConfig() Config {
+	return Config{
+		Connections: 5,
+	}
+}
+
+func init() {
+	options.Register("azure", Config{})
+}
+
+// ParseConfig parses the string s and extracts the azure config. The
+// configuration format is azure:containerName:/[prefix].
+func ParseConfig(s string) (interface{}, error) {
+	if strings.HasPrefix(s, "azure:") {
+		s = s[6:]
+	} else {
+		return nil, errors.New("azure: invalid format")
+	}
+	// use the first entry of the path as the container name and the
+	// remainder as prefix
+	path := strings.SplitN(s, ":/", 2)
+	return createConfig(path)
+}
+
+func createConfig(p []string) (interface{}, error) {
+	if len(p) < 2 {
+		return nil, errors.New("azure: invalid format, container name not found")
+	}
+	cfg := NewConfig()
+	cfg.Container = p[0]
+	if p[1] != "" {
+		cfg.Prefix = path.Clean(p[1])
+	}
+	return cfg, nil
+}