Reduce image size by removing the php source

[fabiang]

Hello,

I've reduced the size of all images by around 10 MB by removing the source code of PHP after compiling it.

To download and delete and the source code again I've added the sub-commands download and delete-download to docker-php-source script, which are called when docker-php-source extract and docker-php-source delete are executed. This allows docker-php-ext-install to work as before.

Possible BC breaks:

1. Installs through pecl
2. Compiling extension directly
3. environment variable PHP_URL was removed

For 1. and 2. docker-php-source download must called before and docker-php-source delete-download for cleanup must be called after.

Of cause the overall build time of the php images increased for most people, but should be acceptable.

I've a quite complex image based on this PHP images and it could be build without problems, so I guess most people won't even notice.

[tianon]

This is technically a duplicate of #488, although I think #886 has more useful discussion in it.