Add Bitbucket crawler for Golang dependencies

[timgluz]

According to go-search, there're 2000 (low priority!!) packages hosted on Bitbucket and it would nice if we could pull Go dependencies from there too like we're doing for Github;

[reiz]

@timgluz OK. That's low priority. If we are totally bored and the sun is not shining and there is no beer in the fridge anymore then we can start working on this.