[WIP] Experimental support for stacktrace of github.com/pkg/errors.

[hiroshi]

This is a work in progress PR. Currently implemented only what just we want.

• [x] Let raven.CaptureError(err,...) use stacktrace of err if available.
• [x] reven.RecoveryHandler
• [ ] Support other Capture functions like CapturePanic

May resolve https://github.com/getsentry/raven-go/issues/88

You can use this by replacing import path from github.com/sentry/raven-go to github.com/nota/raven-go.

You can see this branch is stale or not by https://github.com/getsentry/raven-go/network

[connor4312]

It'd be fantastic to get this PR in, even without the other Capture* functions!

[jsm]

Bump on this.