Only use our whitespace-related YASnippet hook on snippets we generate

1. Make yas-after-exit-snippet-hook to buffer-local value.

2. Restrict payas/remove-extra-whitespace execution only calling
   yas/create-php-snippet.

GitHub-Issue: 4
Signed-off-by: Eric James Michael Ritz <lobbyjones@gmail.com>
Add developers’ commentary about `php-auto-yasnippet-executing`

GitHub-Issue: 4
Setup our whitespace-related hook whenever we load YASnippets

This patch first renames the function `payas-yas-hook-setup` to the
name `yas/initialize`.  That is because we now call the function after
loading YASnippets.  To me that makes the function part of the public
API and we have a convention of prefixing ‘yas’ to all public API
functions, to distinguish them from functions never meant for use
outside of the package.

The name ‘yas/initialize’ comes from similarly-named functions in
YASnippet related packages available from MELPA:

    http://melpa.milkbox.net/

Examples: clojure-snippets, go-snippets, and processing-snippets.

Steve Purcell was the one who suggested I look at packages like these
and so his name now appears in the README as a contributor for
providing the original impetus behind this change (and some others).

GitHub-Issue: 4
Do not make `payas/remove-extra-whitespace` a buffer-local hook

Now that the hook is a no-op in most situations (any time not invoked
by our `yas/create-php-snippet` function) we can register the hook
normally.  This addresses a problem I would run into whereby the hook
did nothing at all.  Admittedly this may not be the best change in the
long-term as we want to make sure our hook does not erroneously affect
any other packages that build on top of YASnippets.  But as a
short-term fix this patch looks ok.

GitHub-Issue: 4
Update version number to 2.0.0