Use the cache when the download times out

Attempt to mitigate https://github.com/ocaml/ocaml.org/issues/358
Add (non trackable) social media links

Fixes https://github.com/ocaml/ocaml.org/issues/303
Add Homebrew installation section in install.md
Factorize slightly better the Makefile compilation rules
Update 99problems.md

Implemented sample solution for complete binary trees.
Slight template change for "centered" pages

Alleviate https://github.com/ocaml/ocaml.org/issues/313
(but is not completely satisfactory).
Add a solution to the 99problems page
Small fixes to the previous submitted solution
Change the URL for jquery
tutorial/guidelines: replace improper language

Fixes #351. Also, fix minor grammar errors.