Add "Package class >> load" and supporting methods
Merge branch 'upstream' into pr-728
Fixed *extension category

Importer-Exporter was renamed to Kernel-ImportExport
Fixes #763
added Package >> loadFromNamespace:

... and Package class >> load:fromNamespace:
update API-CHANGES.txt
Merge branch 'upstream' into ext-728

Conflicts:
	API-CHANGES.txt
removed the unnecessary special line for st.Object

This should finalize the 'nil subclass' changes.
Empty ProtoObject

It only has empty class >> initialize.
It's ready to use, methods can be added to it,
Object is its subclass.
AbstractProxy subclass of ProtoObject
Merge pull request #766 from herby/gh-763

Fixes #763 - invalid at:put:s return value put, plus test
Merge pull request #765 from herby/ext-728

Fixes #727
Merge pull request #768 from herby/nil-subclass-2

removed the unnecessary special line for st.Object
Merge pull request #769 from herby/empty-protoobject

Empty protoobject
Adds basic behavior methods to ProtoObject

- Moves basic behavior methods from Object to ProtoObject
- Updates class comments of ProtoObject and Object
- Removes unneeded AbstractProxy
- Makes JSObjectProxy a subclass of ProtoObject