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.
Merge pull request #3996 from gourlaysama/wip/spec

spec: generated TOC, with linkable headers and return to top links