Add `#travel` and `#travel_to` to AS::TestCase

Add `ActiveSupport::Testing::TimeHelpers#travel` and `#travel_to`. These
methods change current time to the given time or time difference by
stubbing `Time.now` and `Date.today` to return the time or date after
the difference calculation, or the time or date that got passed into the
method respectively. These methods also accept a block, which will
return current time back to its original state at the end of the block.

Example for `#travel`:

    Time.now # => 2013-11-09 15:34:49 -05:00
    travel 1.day
    Time.now # => 2013-11-10 15:34:49 -05:00
    Date.today # => Sun, 10 Nov 2013

    travel 1.day do
      User.create.created_at # => Sun, 10 Nov 2013 15:34:49 EST -05:00
    end

Example for `#travel_to`:

    Time.now # => 2013-11-09 15:34:49 -05:00
    travel_to Time.new(2004, 11, 24, 01, 04, 44)
    Time.now # => 2004-11-24 01:04:44 -05:00
    Date.today # => Wed, 24 Nov 2004

    travel_to Time.new(2004, 11, 24, 01, 04, 44) do
      User.create.created_at # => Wed, 24 Nov 2004 01:04:44 EST -05:00
    end

This module is included in `ActiveSupport::TestCase` automatically.