Improve comment headers

I've adjusted the comment headers (and footers) to be aligned with the
commonly accepted standard format and `M-x checkdoc`.
Merge pull request #25 from bbatsov/improve-headers

Improve comment headers
Rename predicate functions according to Emacs Lisp style

In Emacs predicate should name like something-p instead of
is-something. I've updated a few predicate functions and added them
proper docstrings that pass `M-x checkdoc`.

I've also updated the comment header of the tests file.