MGO-141 implement watch function

Author: Will Banfield

changestreams.go

@@ -39,6 +39,37 @@ type ChangeStreamOptions struct {
 	Collation *Collation
 }
 
+// Watch constructs a new ChangeStream capable of receiving continuing data
+// from the database.
+func (coll *Collection) Watch(pipeline interface{},
+	options ChangeStreamOptions) (*ChangeStream, error) {
+
+	if pipeline == nil {
+		pipeline = []bson.M{}
+	}
+
+	pipe := constructChangeStreamPipeline(pipeline, options)
+
+	pIter := coll.Pipe(&pipe).Iter()
+
+	// check that there was no issue creating the iterator.
+	// this will fail immediately with an error from the server if running against
+	// a standalone.
+	if err := pIter.Err(); err != nil {
+		return nil, err
+	}
+
+	pIter.isChangeStream = true
+
+	return &ChangeStream{
+		iter:        pIter,
+		collection:  coll,
+		resumeToken: nil,
+		options:     options,
+		pipeline:    pipeline,
+	}, nil
+}
+
 // Next retrieves the next document from the change stream, blocking if necessary.
 // Next returns true if a document was successfully unmarshalled into result,
 // and false if an error occured. When Next returns false, the Err method should