Digitalocean provider (#171)

* Implement Digitalocean provider
* Vendor digitalocean lib
* Enable SRV for Digitalocean and fix the tests
* Test cname etc. records pointing to the same domain

vendor/github.com/digitalocean/godo/errors.go

@@ -0,0 +1,24 @@
+package godo
+
+import "fmt"
+
+// ArgError is an error that represents an error with an input to godo. It
+// identifies the argument and the cause (if possible).
+type ArgError struct {
+	arg    string
+	reason string
+}
+
+var _ error = &ArgError{}
+
+// NewArgError creates an InputError.
+func NewArgError(arg, reason string) *ArgError {
+	return &ArgError{
+		arg:    arg,
+		reason: reason,
+	}
+}
+
+func (e *ArgError) Error() string {
+	return fmt.Sprintf("%s is invalid because %s", e.arg, e.reason)
+}