Check spelling

[Peque]

Add a spell checker in Travis.

It should work for both code and comments.

Check old Linux spell checker:

before_script:
  - linux="https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git"
  - wget $linux/plain/scripts/spelling.txt
  - wget $linux/plain/scripts/checkpatch.pl
  - chmod +x checkpatch.pl

script:
  - options="--terse --no-tree --max-line-length=80 --ignore ARRAY_SIZE"
  - ./checkpatch.pl $options --file src/*

Also:

• https://github.com/lucasdemarchi/codespell
• https://github.com/lyda/misspell-check

Maybe if we integrate a new tool for checking misspelling we could remove checkpatch.pl from Travis completely. Check if other errors (i.e.: comment errors) are covered with clang-format as well before.