Add Bitbucket integration

[asabil]

No description provided.

[bradrydzewski]

thanks @asabil

can you please slightly refactor the code, based on my initial commit:
https://github.com/bradrydzewski/drone/blob/bitbucket/pkg/handler/bitbucket.go

the bitbucket handlers should be in a struct called BitbucketHandler and all contained in a single bitbucket.go file. We will be refactoring the github code to be contained in a github.go file as well.

[bradrydzewski]

Also we'll need test coverage. You can see the GitHub tests as reference:
https://github.com/drone/drone/blob/master/pkg/handler/testing/github_test.go

Sorry, I should have been more specific in the Bitbucket thread what still needed to be done

[asabil]

No worries, i will try to make these changes as soon as possible. My fault really didn't check if there was an issue already open

[bradrydzewski]

ah, got it, for some reason I thought that "PUT" worked ... we've been using it for 1+ years and it has been broken this whole time!

[bradrydzewski]

oops, I meant to merge your change to go-bitbucket ... no worries, let's still make the proposed changes if you don't mind.

[bradrydzewski]

fyi, I also added my notes to issue #2 so that we could track progress. I'll close issue #2 and update the documentation once the above changes are made. Thanks again!