Applying the bintray release plugin

applies the release plugin to following modules :
- core
- pxfetcher

and declares the pxfetcher dependencies on core via mavenCentral because of the multiproject issue with `bintray-release`
yeah but not yet, this PR is just to apply the release plugin which I'll then do a release for 1.6.1 to match the current release in maven central but in jcenter