The Great Rewrite (fixes #36, #61, #94, #101)

Rewrite of the file model and pulling mechanism. Needs lots of cleanup
and bugfixes, now...

lamport/clock.go

@@ -0,0 +1,24 @@
+package lamport
+
+import "sync"
+
+var Default = Clock{}
+
+type Clock struct {
+	val uint64
+	mut sync.Mutex
+}
+
+func (c *Clock) Tick(v uint64) uint64 {
+	c.mut.Lock()
+	if v > c.val {
+		c.val = v + 1
+		c.mut.Unlock()
+		return v + 1
+	} else {
+		c.val++
+		v = c.val
+		c.mut.Unlock()
+		return v
+	}
+}