py80211: set policy for WIPHY_BANDS attribute

nl80211_policy does not specify all attributes, which makes them
default to NLA_UNSPEC. Add NLA_NESTED type for the WIPHY_BANDS
attribute.

Signed-off-by: Arend van Spriel <aspriel@gmail.com>
py80211: separate object string representation

The py80211 objects are used in a scripted environment and
there may not be an immediate need to have a string representation
of it. By seperating this from the object the user can choose if
he wants it or not.

Signed-off-by: Arend van Spriel <aspriel@gmail.com>
py80211: add example folder with wiphy retrieval example

Having an example may help understand the python library so
here is a first one, which list wiphy band related information.

Signed-off-by: Arend van Spriel <aspriel@gmail.com>
py80211: remove test code from library file

With examples folder introduced remove test code from the
python library file.

Signed-off-by: Arend van Spriel <aspriel@gmail.com>