Rebind `forward-sexp' and `backward-sexp'

These two functions had been hidden by `back-button-global-forward' and
`back-button-global-backward'.
Do not call (ac-config-default)

It adds extra ac-sources to cc-mode which are not needed.
Add package `nlinum'

Let's disable linum and give nlinum a try.
Add package `auto-complete-clang-async'
Let flycheck clang check against C++11 standard