Encapsulate release notes gen, 2.10.4-RC1 data

It also doesn't actually download the files anymore,
just parses the Content-Length part of the header (asynchronously).


For example:
```
sbt -Dfile.encoding=UTF-8 console
scala> MakeReleaseNotes.genPR("2.10.3", "2.10.4-RC1", "2013/12/20")
```

The output is copy/pasteable in bash to create your scala/scala-lang PR.


/cc @retronym, @gkossakowski