Add repos/sources to get src

Hi, Mike

I added repos/sources to get source file or directory from BitBucket

### Usage

```ruby
@client.repos.sources.find(:username, :reponame, :sha, :path)
```

Actually it is for the first time to do pull request, so I don't know how to remove first two commits...

I'm sorry, but I'm glad to contribute to your great repository.