Add initial support for PHP7 (#97)

* Add initial support for PHP7

* Update PHP env with now an PSR3 logger (Monolog). Now you can also create a handler function with PSR7 Http message and PSR3 logger in parameter.

* Add examples for PHP7 environement

* For development you can create a function in this path /app/userfuncdev.php it will be loaded

* Switch to alpine base image

* handler now receive an array with request, response and logger object

environments/php7/README.md

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+# Fission: PHP Environment
+
+This is the PHP environment for Fission.
+
+It's a Docker image containing a PHP 7.1 runtime. This image use php:7.1-cli base image with the built-in PHP server
+
+This environment didn't force you to use class or create a main function.
+
+A few common extensions are included :
+- curl
+- gd
+- json
+- mcrypt
+- PDO (pdo_mysql, pdo_pgsql, pdo_sqlite)
+- OPcache
+- ftp
+- iconv
+- phar
+- mysqli
+- pgsql
+- SimpleXML
+- xmlrpc
+- zip
+
+## Customizing this image
+
+To add other extensions or packages(composer.json) you need to edit the Dockerfile and rebuild this image (instructions below).
+
+## Rebuilding and pushing the image
+
+You'll need access to a Docker registry to push the image: you can
+sign up for Docker hub at hub.docker.com, or use registries from
+gcr.io, quay.io, etc.  Let's assume you're using a docker hub account
+called USER.  Build and push the image to the the registry:
+
+```
+   docker build -t USER/php7-env . && docker push USER/php7-env
+```
+
+## Using the image in fission
+
+You can add this customized image to fission with "fission env
+create":
+
+```
+   fission env create --name php7 --image USER/php7-env
+```
+
+Or, if you already have an environment, you can update its image:
+
+```
+   fission env update --name php7 --image USER/php7-env
+```
+
+After this, fission functions that have the env parmeter set to the
+same environment name as this command will use this environment.