Fixes for pybind

Fixes for:
- Redefining built-in 'IOError'
- remove unnecessary semicolons
- add a lot of docstrings
- unify make_ex() code in cephfs.py and rados.py
- chmod -x on rados.py

And fixing a bug in rados.py:create_pool() :
Call rados_pool_create_with_all() only if auid and crush_rule are set properly. In case only crush_rule is set call rados_pool_create_with_crush_rule() on librados, not the other way around.