Move backend compilation utilities (#400)

* Move copyResourceToFile() to BackendCompilationUtilities.
* Move BackendCompilationUtilities into a firrtl util package.
Some of this could be moved into a more general tools package, but since chisel3 already has a dependency on firrtl ...
* Push util down into firrtl so as not to conflict with scala.util.
* Use new createTestDirectory. Fixes #452.