Automatically delete .js and .js.map files for deleted classes.

The sbt plugin now patches the IncOptions of Scala.js projects,
so that, when a .class file must be deleted, so are the
corresponding .js and .js.map files.
Exclude parallel collections and util.parsing from stdlib.

Or how to lose 8 MB of uncompressed code and > 50 % of
compressed code with 3 lines in the build file.
Add Scala.js test suite.
Use explicit naming in enumeration test.

Fixes #38