Extended stacktrace output

[davecheney]

This PR introduces an "extended" stacktrace output to this package's error implementation controlled by the %+v verb.

An example of the %+v syntax, taken from ExampleWrap

hello world
github.com/pkg/errors_test.ExampleCause_extended.func1.1
        /home/dfc/src/github.com/pkg/errors/example_test.go:156
github.com/pkg/errors_test.ExampleCause_extended.func1
        /home/dfc/src/github.com/pkg/errors/example_test.go:157
github.com/pkg/errors_test.ExampleCause_extended
        /home/dfc/src/github.com/pkg/errors/example_test.go:158
testing.runExample
        /home/dfc/go/src/testing/example.go:114
testing.RunExamples
        /home/dfc/go/src/testing/example.go:38
testing.(*M).Run
        /home/dfc/go/src/testing/testing.go:744
main.main
        github.com/pkg/errors/_test/_testmain.go:104
runtime.main
        /home/dfc/go/src/runtime/proc.go:183
runtime.goexit
        /home/dfc/go/src/runtime/asm_amd64.s:2059
github.com/pkg/errors_test.ExampleCause_extended
        /home/dfc/src/github.com/pkg/errors/example_test.go:158: failed

The output takes some explaining.

• The first line hello world is the original text passed to errors.Errorf
• The second til the third to last are the stacktrace recorded at the time errors.Errorf was invoked.
• The Last two lines are the stack trace at the point errors.Wrap was called to wrap the previous error with the additional text failed. The stacktrace is not printed in this case, only the point at which the caller was invoked. Printing the additional stack trace is subject for further debate.

The PR is large, but that is due to the large impact it has on the tested output, the actual change to errors.go is 3 lines.

[davecheney]

The tests are failing because I (stupidly) used the paths to my local gopath in the fixtures. I'll fix it tomorrow.

[davecheney]

/cc @ChrisHines all the tests pass again, ready for review.