Add support for managing JS global variables needed by the compiler

This is a first step (or maybe workaround) to solve issue #390.

The following methods have been added:

* `Smalltalk>>#globalJsVariables`
* `Smalltalk>>#addGlobalJsVariable:`
* `Smalltalk>>#deleteGlobalJsVariable:`

Amberc now supports a `-g global1,global2` flag.

The amberc Grunt task now has an optional 'jsGlobals: ['global1', 'global2'],` option for each task.

The names are open for discussion.