Fix #225: provide an equivalent of JS' `typeof x`.

It is introduced as `js.typeOf(x)`.
Merge pull request #236 from sjrd/equivalent-of-typeof-in-package

Fix #225: provide an equivalent of JS' `typeof x`.
Merge pull request #234 from jonas/js-symbol-cache

Add Scala.js specific scala.Symbol implementation
Add implementation for String.equalsIgnoreCase

Also adds missing tests for String.compareToIgnoreCase.

Fixes #233