Add ability to intersect schedules

[craiglittle]

An intersection of two schedules consists of:

• an intersection of intervals
• a union of holidays
• the time zone of the first schedule

For example:

schedule_1 = Biz::Schedule.new do |config|
  config.hours = {
    mon: {'09:00' => '17:00'},
    tue: {'10:00' => '16:00'},
    wed: {'09:00' => '17:00'},
    thu: {'10:00' => '16:00'},
    fri: {'09:00' => '17:00'},
    sat: {'11:00' => '14:30'}
  }

  config.holidays = [Date.new(2016, 1, 1), Date.new(2016, 12, 25)]

  config.time_zone = 'Etc/UTC'
end

schedule_2 = Biz::Schedule.new do |config|
  config.hours = {
    sun: {'10:00' => '12:00'},
    mon: {'08:00' => '10:00'},
    tue: {'11:00' => '15:00'},
    wed: {'16:00' => '18:00'},
    thu: {'11:00' => '12:00', '13:00' => '14:00'}
  }

  config.holidays = [
    Date.new(2016, 1, 1),
    Date.new(2016, 7, 4),
    Date.new(2016, 11, 24)
  ]

  config.time_zone = 'America/Los_Angeles'
end

schedule_1 & schedule_2

results in a schedule that is configured like:

Biz::Schedule.new do |config|
  config.hours = {
    mon: {'09:00' => '10:00'},
    tue: {'11:00' => '15:00'},
    wed: {'16:00' => '17:00'},
    thu: {'11:00' => '12:00', '13:00' => '14:00'}
  }

  config.holidays = [
    Date.new(2016, 1, 1),
    Date.new(2016, 7, 4),
    Date.new(2016, 11, 24),
    Date.new(2016, 12, 25)
  ]

  config.time_zone = 'Etc/UTC'
end

Closes #50.

@zendesk/darko

/cc @pocman

[alex-stone]

👍 looks good to me! Union time?