Moves JCenter and Bintray repo into object Resolver. #1405

This is a belated amendment to #1405.

This is more consistent with other hardcoded repos like Sonatype.
Also I'm hardcoding everything to https. Especially for convenience methods like these, we should be encouraging people to use https out of the box.

build.sbt:

```scala
resolvers += Resolver.bintrayRepo("foo", "bar")
```

/review @jsuereth, @evgeny-goldin