Make schedules easier to set up

Fixes #146

lib/ice_cube/schedule.rb

@@ -21,6 +21,7 @@ def initialize(start_time = nil, options = {})
       self.end_time = options[:end_time] if options[:end_time]
       @all_recurrence_rules = []
       @all_exception_rules = []
+      yield self if block_given?
     end
 
     # Set start_time

spec/examples/schedule_spec.rb

@@ -4,6 +4,14 @@
 
   include IceCube
 
+  it 'yields itself for configuration' do
+    t1 = Time.utc(2013, 2, 12, 12, 34 ,56)
+    schedule = IceCube::Schedule.new do |s|
+      s.start_time = t1
+    end
+    schedule.start_time.should == t1
+  end
+
   describe :duration do
 
     it 'should be based on end_time' do