Remove metadata action, capture errors for http status, extract logging processor
Introduce ContentNegotiation to be able to choose json transport
Bring back metadata as separate middleware
Start of experimental REST adapter

Given the following service:

```
service Foo {
  string foo()
  void bar(map<string,string> m)
  void baz(string a, string b)
}
```

And the following rackup:

```ruby
use Stark::Rack::REST
use Stark::Rack
use Foo::Processor
run FooHandler.new
```

- GET /foo calls #foo()
- GET /bar?a=1&b=2 calls #bar({"a":"1","b":"2"})
- GET /baz?arg[0]=1&arg[1]=2 calls #baz("1", "2")
- All calls return JSON
Add Rack as dev dependency, update readme and manifest
Unmarshal thrift json into a friendlier value