Add SRV Record Type (#136)

* Add support for SRV records for NAMEDOTCOM and ROUTE53.
* Improve docs
* Rename RR() to ToRR().
* Rename RecordConfig Priority to MxPreference (affects json IR data)

providers/diff/diff.go

@@ -33,11 +33,7 @@ type differ struct {
 
 // get normalized content for record. target, ttl, mxprio, and specified metadata
 func (d *differ) content(r *models.RecordConfig) string {
-	content := fmt.Sprintf("%s %d", r.Target, r.TTL)
-	if r.Type == "MX" {
-		content += fmt.Sprintf(" priority=%d", r.Priority)
-	}
-
+	content := fmt.Sprintf("%v ttl=%d", r.Content(), r.TTL)
 	for _, f := range d.extraValues {
 		for k, v := range f(r) {
 			content += fmt.Sprintf(" %s=%s", k, v)

providers/diff/diff_test.go

@@ -94,8 +94,8 @@ func TestMxPrio(t *testing.T) {
 	desired := []*models.RecordConfig{
 		myRecord("www MX 1 1.1.1.1"),
 	}
-	existing[0].Priority = 10
-	desired[0].Priority = 20
+	existing[0].MxPreference = 10
+	desired[0].MxPreference = 20
 	checkLengths(t, existing, desired, 0, 0, 0, 1)
 }