setup.py: update copyright and metadata

Signed-off-by: David Aguilar <davvid@gmail.com>
setup.py: move version information to version.py

Centralize version information into a jsonpickle/version.py
file.  Avoid sandbox violations by reading and executing the
file directly instead of importing it, which avoids pulling in
jsonpickle/__init__.py.

This is suggestion number 1 from @jaraco -- store the version
in a version.py file.

C.f. https://github.com/jsonpickle/jsonpickle/issues/20

Closes #20

Helped-by: Jason R. Coombs <jaraco@jaraco.com>
Signed-off-by: David Aguilar <davvid@gmail.com>