stmhal: Fix ptr arith in CC3000 code; enable network build in travis.
py: mp_obj_str_get_str(): Work with bytes too.

It should be fair to say that almost in all cases where some API call
expects string, it should be also possible to pass byte string. For example,
it should be open/delete/rename file with name as bytestring. Note that
similar change was done quite a long ago to mp_obj_str_get_data().
objstr: Allow to convert any buffer proto object to str.

Original motivation is to support converting bytearrays, but easier to just
support buffer protocol at all.