Preinstall some common PHP extensions.

Introduces the php::extensions recipe and the php_pecl resource. The
recipe uses nodes.php.multi.extensions attributes to install extensions
to multiple installed PHP versions via the php_pecl resource. The
php_pecl resource can take the following parameters:

* extension: the extension's name.
* channel: an optional channel which has to be discovered for the
  extension to be installed via PECL.
* versions: the PHP versions for which the extension is gonna be
  installed.
* before_packages: optional packages required by the extension.
* before_script: an optional script that gonna be executed before
  installing the extension.

For now and according to travis-ci/travis-ci#511, the following
extensions are configured in `php/attributes/default.rb`:

* apc
* memcache
* memcached
* mongo
* amqp
* zmq
Improve error detection on PECL extension installation.
PEAR and PECL are not available on 5.5 for now.
Preinstalled PECL extensions are not enabled by default.
Install phpenv along with config-add and config-rm extensions.
Update libmemcached to 1.0.16.
Merge branch 'php_extensions'