Replace Getter and Setter JS tree by Property tree

Implement property desugaring and printing
Remove compiler internal dependency on ScalaJS.classes
Fix #276: Implement @JSExport

This commit completely removes bridges and replaces them by explicit
exports through the @JSExport annotation.
Merge pull request #314 from gzm0/jsexport

Add support for JSExport. Drop bridges
Merge branch 'master', remote-tracking branch 'scala-js/master'