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.
Merge pull request #26 from bbatsov/master

Update a few predicate functions according to the common Emacs Lisp style