+build #158 make artifacts include osgi MANIFEST

So sadly I was not able to declare this at the top-level (less repetition), as the plugin blows up in weird ways (internets point out it [has trouble being declared in a project without sources](https://issues.gradle.org/browse/GRADLE-2391) (as in, our root project I guess)).

Example MANIFEST.MF looks like:

```
Manifest-Version: 1.0
Bundle-Description: Reactive Streams API
Export-Package: org.reactivestreams
Bundle-SymbolicName: org.reactivestreams.reactive-streams-api
Bundle-Version: 1.0.0.M1
Bundle-Name: reactive-streams-api
Bundle-ManifestVersion: 2
Bnd-LastModified: 1416835216000
Bundle-Vendor: Reactive Streams SIG
Bundle-DocURL: http://reactive-streams.org
Created-By: 1.8.0 (Oracle Corporation)
Tool: Bnd-2.1.0.20130426-122213
```

Resolves blocker issue for Slick (they have to publish osgi artifacts), cc @szeiger.
As I'm no expert on osgi things, please confirm that's all an osgi bundle needs.

Resolves https://github.com/reactive-streams/reactive-streams/pull/159