Allow PostBodyContentTypeParser Middleware to use a custom json parser

I added the ability to add a custom json parser to the PostBodyContentTypeParser middleware. It now takes a block that passes in the request body, there you can call `Oj.load(body)` or `Yajl::Parser.parse(body)` depending on the parser you want to use. Here's an example:

```ruby
use Rack::PostBodyContentTypeParser do |body|
  Oj.load(body)
end
```

I couldn't get the test suite to run at all locally, so I hope the tests pass.