spec: generated TOC with linkable headers

This adds a table of contents to the spec, with a way to get the link of a
header on hover (like on GitHub) and a "return to top" small icon at the
end of each section.

Jekyll generates the H1-level table of contents, and then the current
one is expended using jQuery all the way down to H5 titles. Examples and
H6 headers are ignored.

GitHub's (MIT licensed) octicons are used for nice "link" and "up"
icons.
spec: a print stylesheet to remove the TOC when printing
spec: a header that links to the index, with a scala logo

...and a Scala favicon.
Bump timeout in interactive tests from 10s -> 30s.

Seeing too many of these failures on our build servers.

https://scala-webapps.epfl.ch/jenkins/view/2.N.x/job/scala-nightly-auxjvm-2.11.x/148/jdk=jdk8,label=auxjvm/console

This is most likely due to load on the machines.
Cleanup a few flags in test/files/

 * String interpolation isn't Xexperimental anymore

   A few useless Xexperimental flags in tests were left behind by 6917cca,
   after string interpolation was made non-experimental in 983f414.

 * things added under -Xfuture in 2.10 are very much Xpresent now, the
   flag isn't needed anymore.
Merge pull request #4007 from retronym/topic/bump-timeout

Bump timeout in interactive tests from 10s -> 30s.
Merge pull request #4009 from gourlaysama/wip/test-flags

Cleanup a few compiler flags in test/files/
Merge pull request #3996 from gourlaysama/wip/spec

spec: generated TOC, with linkable headers and return to top links
Merge remote-tracking branch 'origin/2.11.x' into merge/2.11.x-to-2.12.x-20140930