add scala.concurrent.java8.FutureConverter

Conversions between Future and CompletionStage, with JavaDoc. Also
contains accessors for ExecutionContext converters and Promise
construction (which are basically inaccessible in the Scala library
because they are located in companion objects to traits).
add Scala extension methods for Future conversions

also remove some duplicated dependency on JUnit
Merge pull request #7 from rkuhn/wip-future-converters

add scala.concurrent.java8.FutureConverter