All Vendor dir should be ignored

As it's composer's install dir, it should be ignored by default, and
if
someone wants to manually add a vendor, git add --force should be
used
overwhise the Vendor/xxx list in the .gitignore can be very long
depending on how many composer package you use
Include composer's autoloader

As describe in
http://book.cakephp.org/2.0/en/installation/advanced-installation.html
Overwhise, none of the packages installed with composer will be
accessible
Added phpunit as dev requirement

Composer is used to not depend of external environment, if it's not
defined as require-dev, system installed phpunit will be used, which
should not be, or in the worst case phpunit is not installed globally
and tests cannot be runned
Conditionally loads composer's autoload.php

We also remove and re-prepend CakePHP's autoloader as composer thinks it is
the most important.
See composer/composer@c80cb76b9b5082ecc3e5b53b1050f76bb27b127b
Merge branch 'pr/5'