Added preliminary PHP support.

PHP support is work-in-progress. It allows deploying of PHP web applications from git repositories and tarball sources. 

Below, we can see a  php deployment example using a git repo.

```
name: HelloWorld
services:
- serviceType: brooklyn.entity.webapp.apache.ApacheServer
 name: Apache Server
 location: localhost
 http_port: 80
 app_git_repo_url: https://github.com/kiuby88/phpHelloWorld.git
```

The next plan shows a php deployment example using a tarball resource

```
name: HelloWorld
services:
- serviceType: brooklyn.entity.webapp.apache.ApacheServer
 name: Apache Server
 location: localhost
 http_port: 80
 app_tarball_url : https://github.com/kiuby88/phpHelloWorld.git 
```

Known issues:
- This implementation deploys an Apache Web Server. No application server support yet. 
- Metrics will be provided by Apache mod_status module.
- No HTTPS support yet.

I'm not completely sure HttpServer should always extend PhpWebApp*
Unless it is a customized entity for Seaclouds, we should make the PHP support optional

@ahgittin what do you think?