add modify-increment extension support detailed in RFC4525 (#227)

* add modify-increment extension support detailed in RFC4525

This extension is detailed in [RFC4525](https://tools.ietf.org/html/rfc4525).
To summarize, it allows for auto-incrementing a specific attribute by a
specific amount. It can be used in conjunction with pre-read, post-read,
and assert control types

* update v3

modify.go

@@ -33,9 +33,10 @@ import (
 
 // Change operation choices
 const (
-	AddAttribute     = 0
-	DeleteAttribute  = 1
-	ReplaceAttribute = 2
+	AddAttribute       = 0
+	DeleteAttribute    = 1
+	ReplaceAttribute   = 2
+	IncrementAttribute = 3 // (https://tools.ietf.org/html/rfc4525)
 )
 
 // PartialAttribute for a ModifyRequest as defined in https://tools.ietf.org/html/rfc4511
@@ -97,6 +98,11 @@ func (req *ModifyRequest) Replace(attrType string, attrVals []string) {
 	req.appendChange(ReplaceAttribute, attrType, attrVals)
 }
 
+// Increment appends the given attribute to the list of changes to be made
+func (req *ModifyRequest) Increment(attrType string, attrVal string) {
+	req.appendChange(IncrementAttribute, attrType, []string{attrVal})
+}
+
 func (req *ModifyRequest) appendChange(operation uint, attrType string, attrVals []string) {
 	req.Changes = append(req.Changes, Change{operation, PartialAttribute{Type: attrType, Vals: attrVals}})
 }

v3/modify.go

@@ -33,9 +33,10 @@ import (
 
 // Change operation choices
 const (
-	AddAttribute     = 0
-	DeleteAttribute  = 1
-	ReplaceAttribute = 2
+	AddAttribute       = 0
+	DeleteAttribute    = 1
+	ReplaceAttribute   = 2
+	IncrementAttribute = 3 // (https://tools.ietf.org/html/rfc4525)
 )
 
 // PartialAttribute for a ModifyRequest as defined in https://tools.ietf.org/html/rfc4511
@@ -97,6 +98,11 @@ func (req *ModifyRequest) Replace(attrType string, attrVals []string) {
 	req.appendChange(ReplaceAttribute, attrType, attrVals)
 }
 
+// Increment appends the given attribute to the list of changes to be made
+func (req *ModifyRequest) Increment(attrType string, attrVal string) {
+	req.appendChange(IncrementAttribute, attrType, []string{attrVal})
+}
+
 func (req *ModifyRequest) appendChange(operation uint, attrType string, attrVals []string) {
 	req.Changes = append(req.Changes, Change{operation, PartialAttribute{Type: attrType, Vals: attrVals}})
 }