Adding lazy service

This pr aims to fix #66. The problem is that [FinatraServer](https://github.com/capotej/finatra/blob/master/src/main/scala/com/twitter/finatra/FinatraServer.scala) assigns the value of [service](https://github.com/capotej/finatra/blob/master/src/main/scala/com/twitter/finatra/FinatraServer.scala#L63) during construction, which doesn't leave any chance for the additional filters that are added later to be taken into account. Making this field ```lazy``` fixes the problem. 

Testing this change without exposing the [service](https://github.com/capotej/finatra/blob/master/src/main/scala/com/twitter/finatra/FinatraServer.scala#L63) turned out to be difficult. I tried my best but I am aware that the test is not the healthiest one.