Add g_input_stream_read_all_async()

Add an asynchronous version of _read_all().

This API is not fully consistent with the normal expectations of a
non-asynchronous version.  Consistency between the sync and async version is
probably more important.

The API will still bind correctly, but access to all functionality will
not be available: specifically, in the case of an error, higher level
languages will be unable to determine how many bytes were successfully
read before the error.  Most users will probably not want to use this
information anyway, so this is OK -- and if they do need the
information, then they can just write the loop for themselves.

Heavily based on a patch from Ignacio Casal Quinteiro.

https://bugzilla.gnome.org/show_bug.cgi?id=737451
Add g_output_stream_write_all_async()

Similar to the previous patch, this commit contains a minor violation of
normal API conventions.  See the explanation in the previous commit
message.

Heavily based on a patch from Ignacio Casal Quinteiro.

https://bugzilla.gnome.org/show_bug.cgi?id=737451
docs: explain inconsistency of _{read,write}_all()

These functions are inconsistent with our normal conventions in that
they set an output variable to a specified value, even in the case that
an error is thrown.

Document very clearly that this should be considered exceptional.

https://bugzilla.gnome.org/show_bug.cgi?id=737451
Add tests for {read,write}_all_async()

https://bugzilla.gnome.org/show_bug.cgi?id=737451