Cross compile projects for Scala and Scala.js

[vic]

Allows using goggles from Scala.js projects. I know a previous scala.js branch was reverted #44 due to it altering the project layout, on this PR I'm trying to keep things on their original place while still compiling to both platforms.

Now that Scala.js 1 is on the wild it's much more easier to cross compile this project without changing the project structure.

• Added plugin sbt-crossproject to target Scala 1.2.0
• Configured build.st for cross compilation
  root projects have the setting withoutSuffixFor(JVMPlatform) so that you can use them directly without prefix

  sbt> dslProject/test
  ...
  [info] Passed: Total 141, Failed 0, Errors 0, Passed 141
  
  sbt> dslProjectJS/test
  [info] Passed: Total 141, Failed 0, Errors 0, Passed 141
  

• Tests for dslProject are green on both JS and JVM platform
• Tests for macrosProject are green and only run on JVM platform since they are a compile-time feature.
• Upgraded specs2 to version 4.10.3 which fully supports scalajs1.
• Upgraded monocle-core to version 1.7.3 (latest major 1.x version that uses scalaz) that supports scalajs1
• Added a jitpack.yml file so that we can fetch cross compiled artifacts from jitpack.io for no-yet-released branches like this one.

[vic]

Tests green running on GitHub CI via #48