Adding png support

I have made a couple big changes in this request for you to review

1. i am sending all content as base64 encoded and decoding in the /pastie action. this allows for sending binary data and i don't think it really affects string data.
2. i modified the is_html check, and i don't really understand what the ~ does, so i am not sure how my change affects that line.

I have plans to abstract this out to allow for any image format (jpg and gif), but will get to that over the weekend.

Finally, I have no idea how to not submit the "merge branch" as part of this pull request.

Also, this is my first pull request ever, so let me know if I have done anything wrong.