Show seed at start of suite run

Have the reporter notify formatters of the seed used on `start`
Attempt at tackling #1719 

This can use the `an_object_having_attributes` matcher (an alias of `have_attributes`) rather than a block:

``` ruby
expect(formatter).to receive(:seed).with(
  an_object_having_attributes(:seed => config.seed, :seed_used? => config.seed_used?)
)