general: adding the Clock interface to aid testing

This commit adds Clock and DefaultClock and moves the private
invoices.testClock under the clock package while adding basic
unit tests for it.
Clock is an interface currently encapsulating Now() and TickAfter().
It can be added as an external dependency to any class. This way
tests can stub out time.Now() or time.After().

The DefaultClock class simply returns the real time.Now() and
time.After().

clock/default_clock.go

@@ -0,0 +1,24 @@
+package clock
+
+import (
+	"time"
+)
+
+// DefaultClock implements Clock interface by simply calling the appropriate
+// time functions.
+type DefaultClock struct{}
+
+// NewDefaultClock constructs a new DefaultClock.
+func NewDefaultClock() Clock {
+	return &DefaultClock{}
+}
+
+// Now simply returns time.Now().
+func (DefaultClock) Now() time.Time {
+	return time.Now()
+}
+
+// TickAfter simply wraps time.After().
+func (DefaultClock) TickAfter(duration time.Duration) <-chan time.Time {
+	return time.After(duration)
+}