clock, timecop, table driven tests

• add the ability to manipulate observed time through the clock and timecop package
• add experimental spec helpers to the httpspec
• add support to simulate random io reads with random.Random
• add the ability to pretty print any object, diff them in GNU side-by-side Diff style
• support accessing a testcase.Var's Super value when you define it in a sub context
  • this allows you to wrap entities with Spies and Stubs to do various things, such as fault injection.
• testcase.LetValue accepts more value as long they are safe to use due to pass-by-value.
• support table tests with table test helper.

Happy testing!