Split the project into multiple modules

My attempt at splitting the project into multiple modules. All dependency versions were left unchanged. The following modules were created:
* newman-core
* newman-twitter
* newman-apache
* newman-finagle
* newman-spray
* newman-caching
* newman-serialization

I couldn't work out a way to have one import for the dsl so it currently works like
```scala
 import com.stackmob.newman.dsl.{RichIOHttpResponse=>_, RichFutureHttpResponse=>_, _}
 import com.stackmob.newman.serialization.dsl._
```
which isnt very nice

Any comments appreciated