adding axes argument to mopad Beach

analog to https://github.com/leouieda/obspy/commit/f9e4a4f8f797c9302ce5df371ed8cf0f9c4b57e8
see #734
using native_str for _png module

on python2 _png module seems to accept native strings only. Using future
native_str to use native str (non unicode) for python2 and str (unicode)
for python3.

closes #741.