[WIP] Add ProxyTestCase

This depends on FriendsOfSymfony/FosHttpCache#103. 

It adds:
* ProxyTestCase for functional cache tests in Symfony2 bundles
* A `@clearCache` annotation that will clear the configured proxy before each test case starts. This is automatic in the lib’s `AbstractProxyClientTestCase`, but my experience with Symfony apps is that usually only one or two different test classes are extended, so doing stuff automatically can really be a pain then. It’s better to add some functionality behind a PHPUnit annotation that users can decide when they want the behaviour.
* Some config for starting the proxy server in tests:

```yaml
fos_http_cache:
    proxy_server:
        varnish:
            binary: /usr/sbin/varnishd
            port: 8080
            config_file: /etc/varnish/default.vcl
```

Some problems:
* The `ProxyTestCase` is similar in some ways to the lib’s testcase. However, we cannot easily re-use that (except with traits) because we need to extend Symfony’s `WebTestCase` here for `ProxyTestCase` to have access to the container and be useful in a Symfony app.
ah right. just had a look - does it even make sense to extend WebTestCase with our test case? it could be more confusing than helpfu that createClient and getClient do two completely different things? one starting an in-process symfony where you can mock some services to your gusto, the other running actual curl requests?