Export credentials functions to be able to use them as a library.

Signed-off-by: David Calavera <david.calavera@gmail.com>

credentials/helper.go

@@ -11,8 +11,12 @@ type Credentials struct {
 
 // Helper is the interface a credentials store helper must implement.
 type Helper interface {
+	// Add appends credentials to the store.
 	Add(*Credentials) error
+	// Delete removes credentials from the store.
 	Delete(serverURL string) error
+	// Get retrieves credentials from the store.
+	// It returns username and secret as strings.
 	Get(serverURL string) (string, string, error)
 }