Fix backward incompatibility for Set* methods (#213)

Patch #185 introduced a backward incompatibility by changing the arguments
of the `Set*` methods on `Tree`.

This change restores the arguments to what they previous were, and
introduces `SetWithComment` and `SetPathWithComment` to perform the same
action.

toml.go

@@ -178,14 +178,26 @@ func (t *Tree) GetDefault(key string, def interface{}) interface{} {
 // Set an element in the tree.
 // Key is a dot-separated path (e.g. a.b.c).
 // Creates all necessary intermediate trees, if needed.
-func (t *Tree) Set(key string, comment string, commented bool, value interface{}) {
-	t.SetPath(strings.Split(key, "."), comment, commented, value)
+func (t *Tree) Set(key string, value interface{}) {
+	t.SetWithComment(key, "", false, value)
+}
+
+// SetWithComment is the same as Set, but allows you to provide comment
+// information to the key, that will be reused by Marshal().
+func (t *Tree) SetWithComment(key string, comment string, commented bool, value interface{}) {
+	t.SetPathWithComment(strings.Split(key, "."), comment, commented, value)
 }
 
 // SetPath sets an element in the tree.
 // Keys is an array of path elements (e.g. {"a","b","c"}).
 // Creates all necessary intermediate trees, if needed.
-func (t *Tree) SetPath(keys []string, comment string, commented bool, value interface{}) {
+func (t *Tree) SetPath(keys []string, value interface{}) {
+	t.SetPathWithComment(keys, "", false, value)
+}
+
+// SetPathWithComment is the same as SetPath, but allows you to provide comment
+// information to the key, that will be reused by Marshal().
+func (t *Tree) SetPathWithComment(keys []string, comment string, commented bool, value interface{}) {
 	subtree := t
 	for _, intermediateKey := range keys[:len(keys)-1] {
 		nextTree, exists := subtree.values[intermediateKey]