WIP: JSON-Hal

Here is another implementation of JSON Hal (and XML, YML does not add Links yet). It is based off of master. 

Here is what I have done: 
- Added a `jsonhal` format that serializes with `_links` and `_embedded`
- Added a new Link annotation that looks like the following: 

```
    /*
     * @JMSSerializer\Link(rel = "self", href="/href/here/")
     * @JMSSerializer\Link(rel = "alternative", route = "my_route", parameters = {"id" = "id"})
     */
```

This will add two links where for the `self` link, the URL is simply used verbatim from the `href` parameter, for the second one, the URL is generated with the `route` and the `parameters` (the parameters are used as `PropertyPath`'s. 
- XML serializes links according to the Atom spec (I think).
- YML right now does not serialize the links. 

This is work in progress, I wanted to solicit feedback before putting in more time.

Known issues and todos: 

- I want to add a second `router` service, so that one of them can be replaced for generating templated URLs (RFC6570). I am working on such a router at the moment. I am having a hard time getting the service configured with a parameter though (service name as a parameter, not the class name)
- Unit tests are very minimal right now, a lot of them are emptied out for the JSONHal tests. Most of them most likely just need updated expected results. Some will only work if a Deserializer is implemented. 
- One unit test is failing right now (`JMSSerializerExtensionTest::testLoad`). I don't fully understand why yet. 
- XML link generation may need some tweaking
- YML link generation needs to be implemented. 
- When there are no elements in the relation, the Serializer does not know what `rel` to use for embedding the empty array. Don't really know how to do solve this without explicit type hinting with an annotation.
- Only annotation work right now, XML, YML, and PHP right now is not supported, as I personally don't have a lot of use for that. 
- Documentation

P.S. I know that there is another open PR right now to implement JSON+HAL for the Serializer right now (https://github.com/schmittjoh/JMSSerializerBundle/pull/121). This was based on the 0.9 branch though. Trying to rebase led to massive conflicts, and failed unit tests, so this seemed easier.