Use more display precision for Time and DateTime

The `eq` matcher often results in confusing output when comparing `Time`
and `DateTime` objects.

`Time#inspect` returns a value that is not full precision. This often
leads to a confusing failure message when two time objects differ by
fractional seconds. The matcher returns a failure but the diff shows
identical values. Microseconds have been added to the diff output for
`Time`.

By default `DateTime#inspect` outputs enough precision to prevent this
problem. `ActiveSupport` monkey patches `#inspect` to print a more human
readable but ambiguous representation. If `ActiveSupport` is defined a
custom format string is used for `DateTime` that has enough precision
to differentiate the two objects.

Fixes rspec/rspec-expectations#331
Merge pull request #416 from thomas-holmes/clarify-time-diffs

Use more display precision for Time and DateTime