Add FPM variant

Fixes #10

Size comparisons, for giggles:

```console
$ docker images php
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
php                 5.4                 6b03f428509e        3 hours ago         354.3 MB
php                 5.4-fpm             090b99c15283        15 minutes ago      363.1 MB
php                 5.5                 19a5113df69a        20 minutes ago      356.4 MB
php                 5.5-fpm             8072460e2161        12 minutes ago      365.5 MB
php                 5.6                 69aead156fe7        22 minutes ago      356.8 MB
php                 5.6-fpm             505a5818783e        12 minutes ago      366.1 MB
```