Less Composer dependencies: make Ocramius/ProxyManager (lazy injection) optional

[mnapoli]

The Ocramius/ProxyManager package comes with a lot of dependencies. This package is useful only for lazy injection.

This package will not be installed by default in v5.0 in order to lighten PHP-DI's dependencies.

Before:

• doctrine/annotations
• doctrine/cache
• doctrine/lexer
• container-interop
• phpdocreader
• myclabs/php-enum
• zendframework/zend-stdlib
• zendframework/zend-eventmanager
• zendframework/zend-code
• ocramius/proxy-manager

After:

• doctrine/annotations
• doctrine/cache
• doctrine/lexer
• container-interop
• phpdocreader
• myclabs/php-enum

In order to use lazy injection, users will have to require ocramius/proxy-manager themselves. This will be reflected in the documentation.