setup: update installation infos

- clean up setup.py and MANIFEST.in
- simplejson isn't needed anymore
- allow webob >= 1.0.8 as openstack quantum uses >= 1.0.8 version
- allow gflags >= 1.3 as Fedora 18 uses gflags 1.5

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>