fix a few HTML validity problems suggested by Kurt Hornik

- the align attribute on the td element is obsolete
- cellspaceing on table
- charset on script without src
- DOCTYPE declaration and encoding
use svg instead of png for the travis ci icon
add the alt attribute for <img> to pass the w3c markdup validator
use the rstudio mirror
it seems 3c75f750632bf8e92ec756c9e624247e15cf0697 was carelessly reverted in the PR #823
arrange engines in the alphabetical order, and replace node with javascript in engine_output() instead of only the markdown output hooks

cleans after these two commits:

2daf8940649038a20ea8d56d2fd7080e982994cd
ea95c0d9da02e1bafdefd0843c379064faa03845
a news item for #823
cosmetic
include the solution cache=2 for hook_purl and close #827
fixes #828: consider the pandoc output format in render_markdown()
bump version