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`
I initially did it when everyone was on different versions of AS and tools (shifting it all to gradle.properties). And I tried setting up a Travis project yday, and would have had to maintain my own gradle.properties and Travis env variables.

Instead (post discussion, for benefit of everyone) I would keep these in gradle.properties, and have my API key (which is why I didn't have gradle.properties in Travis) in a _different_ file which is git ignored.