BintrayMavenRepository added

This PR needs some additional work. Right now trying to use sbt 0.13.6-SNAPSHOT I'm getting 

    java.lang.NoSuchFieldError: JCenter
        at sbt.Classpaths$.sbt$Classpaths$$bootRepository(Defaults.scala:1555)
        at sbt.Classpaths$$anonfun$appRepositories$1.apply(Defaults.scala:1527)
        at sbt.Classpaths$$anonfun$appRepositories$1.apply(Defaults.scala:1527)
        at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
        at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
        at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
        at scala.collection.mutable.WrappedArray.foreach(WrappedArray.scala:34)
        at scala.collection.TraversableLike$class.map(TraversableLike.scala:244)
        at scala.collection.AbstractTraversable.map(Traversable.scala:105)
        at sbt.Classpaths$.appRepositories(Defaults.scala:1527)
        at sbt.Classpaths$$anonfun$40.apply(Defaults.scala:1041)
        at sbt.Classpaths$$anonfun$40.apply(Defaults.scala:1041)
        at scala.Function1$$anonfun$compose$1.apply(Function1.scala:47)
        at scala.Function1$$anonfun$compose$1.apply(Function1.scala:47)
        at sbt.EvaluateSettings$MixedNode.evaluate0(INode.scala:175)
        at sbt.EvaluateSettings$INode.evaluate(INode.scala:135)
        at sbt.EvaluateSettings$$anonfun$sbt$EvaluateSettings$$submitEvaluate$1.apply$mcV$sp(INode.scala:69)
        at sbt.EvaluateSettings.sbt$EvaluateSettings$$run0(INode.scala:78)
        at sbt.EvaluateSettings$$anon$3.run(INode.scala:74)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at java.lang.Thread.run(Thread.java:745)
    [error] java.lang.NoSuchFieldError: JCenter
So, having "magic strings" is generally not recommended.   For JCenter, I'd recommend somethign like:

```scala
class JCenter(isSecure: Boolean = false) extends MavenRepository { ..}
class BintrayMavenRepository(subject: String, repo: String) extends MavenRepository { ... }
```

Would be a bit more ideal.