RF: remove circular import in tests

Importing EXT_LIBS from test_delocate causing a potential circular
import revealed by 'nosetests delocate/tests/test_libsana.py'.

Import EXT_LIBS from test_install_names - its origin in any case.
RF: split system library names into own constants

A little easier for libsana testing.
DOC: fix docstrings for copied_libs return arg

copied_libs return arg gives depended_realpath, depdict key value pairs,
but docstring implied depended_realpath, depending_path or something
like.
NF+TST: make prefix strippers public and test

Promote prefix stripper to public name for other modules to use.
Add prefix stripper for realpath + sep case.
Add tests
RF: add fused lib; clean up arch constants

Add fused (dual-arch) library for testing.
Add ARCH_* constants to test_tools and import from there.
Use ARCH_* constants for comparisons
Add test for fused library arch
RF: use frozenset for returned architectures

Frozenset because architectures will not need to be modified, and
because the result can be hashed to be included in sets.
DOC: updating README to current code
RF: clean up some unused imports