Provide the ability to override HTTPTransport configuration

[mattrobenolt]

Fixes #221, #169, #150

edit @kamilogorek: and https://github.com/getsentry/raven-go/issues/117

[dcramer]

I might take a stab at expanding on this since this isn't priority, but I think we'd want to get some basic test coverage in.

[mattrobenolt]

I'm going to change this to not pull in certifi by default as discussed, since this makes it easy to pull in certifi yourself now, and I'll just add documentation on how to do this.

[pedro-te]

This is an issue for me as well. Our sentry server has an SSL cert signed by an internal CA and we need to be able to pass a custom certificate pool. Any chance this can be merged if the conflict is fixed?

Also, the certificate signed by unknown authority error is completely ignored which is frustrating. Spent some time trying to figure out what I was doing wrong because this wasn't throwing any errors until I tried to run the example.go and it threw the certificate signed by unknown authority error.