Tubes with context.encoding

[Arusekk]

This is an example of what can be done. I can work more on the doctests so that they cover the new code.

Closes #1315

[zachriggle]

Thanks for the work on these! I think it might be easier (and more maintainable) to have these declared dynamically.

Consider sticking something at the end of tube.py that looks like this:

for func in [tube.recv, 
             tube.recvn, 
             tube.recvlines, 
             tube.recvuntil]:
    def make_wrapper(func):
        def wrapper(self, *a, **kw):
            return bytearray(func(self, *a, **kw))
        wrapper.__doc__ = 'Same as {func.__name__}, but returns a bytearray'.format(func=func)
        wrapper.__name__ = func.__name__ + 'b'
        return wrapper
    wrapper = make_wrapper(func)
    setattr(tube, wrapper.__name__, wrapper)

# Clean up module scope
del make_wrapper
del wrapper

You can then do the following:

>>> help(tube.recvb)
Help on method recvb in module pwnlib.tubes.tube:

recvb(self, *a, **kw) unbound pwnlib.tubes.tube.tube method
    Same as recv, but returns a bytearray

>>> t=tube()
>>> t.unrecv('asdfasdf')
>>> t.recvnb(3)
bytearray(b'asd')
>>> t.recvnb(3)
bytearray(b'fas')
>>> t.recvnb(3)
EOFError