Scala 2.10.4-RC1

Review by @sjrd please. (BTW, what does the start: header do? is that related to when the post goes live? we had this discussion before, but now I'm not sure anymore.)

Generated with (approximately)

```
hub clone scala/make-release-notes
cd make-release-notes
git checkout 2.10.x
sbt -Dfile.encoding=UTF-8 console
MakeReleaseNotes(new java.io.File("/Users/adriaan/git/scala"), "v2.10.3", "v2.10.4-RC1", MarkDown)
new MakeDownloadPage("2.10.4-RC1", new java.util.Date("20 Dec 2013")).write()
```