unpickler: do not setattr() on tuples

Python 2.7 and newer does not allow doing setattr()
on instances of tuple().  We don't actually need to do it
since the namedtuple has already been completely decoded,
so returning early is desired.

This fixes the tests on Python 2.7.3.

Signed-off-by: David Aguilar <davvid@gmail.com>
tests: simplify tests by moving samples.py to jsonpickle._samples

jsonpickle requires that the objects it reconstitutes are available
globally via a module.

This makes some of the doctests difficult to write because samples.py
is not always available on the sys.path.

Simplify things by moving our samples objects to jsonpickle/_samples.py.
This removes some duplication from the doctests and ensures that
nosetests --with-doctest works correctly without needing to modify
the PYTHONPATH.

Signed-off-by: David Aguilar <davvid@gmail.com>