Introduce Signature

It brings the data into go-land so we don't have to worry about the
commit being there. It stores the data we get from git and provides a
Time() function to get a time.Time struct.

commit.go

@@ -9,6 +9,7 @@ extern int _go_git_treewalk(git_tree *tree, git_treewalk_mode mode, void *ptr);
 import "C"
 
 import (
+	"time"
 )
 
 // Commit
@@ -38,15 +39,35 @@ func (c *Commit) TreeId() *Oid {
 	return newOidFromC(C.git_commit_tree_id(c.ptr))
 }
 
-/* TODO */
-/*
 func (c *Commit) Author() *Signature {
 	ptr := C.git_commit_author(c.ptr)
-	return &Signature{ptr}
+	return newSignatureFromC(ptr)
 }
 
 func (c *Commit) Committer() *Signature {
 	ptr := C.git_commit_committer(c.ptr)
-	return &Signature{ptr}
+	return newSignatureFromC(ptr)
+}
+
+// Signature
+
+type Signature struct {
+	Name string
+	Email string
+	UnixTime int64
+	Offset int
+}
+
+func newSignatureFromC(sig *C.git_signature) *Signature {
+	return &Signature{
+		C.GoString(sig.name),
+		C.GoString(sig.email),
+		int64(sig.when.time),
+		int(sig.when.offset),
+	}
+}
+
+func (sig *Signature) Time() time.Time {
+	loc := time.FixedZone("", sig.Offset*60)
+	return time.Unix(sig.UnixTime, 0).In(loc)
 }
-*/