log test success messages, viewable with go test -v

Hi,

Thanks for the awesome module :-)

This commit calls t.Logf (with hopefully compatible whitespace indentation) for each successful assertion. Without '-v' to 'go test' this should have no effect. With '-v' it shows a log of passing tests - something I have become accustomed to with other test frameworks and something I'd like to use with testify.

If you like the approach, but don't want it "always on", I could make it conditional on some state in the assertion object, but I thought the '-v' switch to 'go test' would already suffice for that.

regards,

jb