Add SBT cookbook in the documentation

First draft, feedback welcome :smile: 

On SBT settings documentation I found:
```scala
confDirectory := "myConfFolder" 
```
I think the documentation is outdated and must be:
```scala
confDirectory := new File("myConfFolder")
``` 
And maybe we can move that part to the cookbook ?

Play SBT settings to cover ?

- [ ] `devSettings`
- [ ] `templatesImport`
- [x] `playAssetsDirectories`
- [ ] `playInteractionMode`
- [ ] `routesImport`
- [x] `ivyLoggingLevel`
- [ ] `testOptions in Test`
- [x] `fork in Test`

RequireJS SBT settings are documented here : http://www.playframework.com/documentation/2.2.x/RequireJS-support

Sources, resources and conf directory ?
```scala
    sourceDirectory in Compile <<= baseDirectory / "app",
    sourceDirectory in Test <<= baseDirectory / "test",

    confDirectory <<= baseDirectory / "conf",

    resourceDirectory in Compile <<= baseDirectory / "conf",

    scalaSource in Compile <<= baseDirectory / "app",
    scalaSource in Test <<= baseDirectory / "test",

    javaSource in Compile <<= baseDirectory / "app",
    javaSource in Test <<= baseDirectory / "test",
```

Ref #2408