Mozilla bug 865244 - Add cubeb_stream_get_max_channels.

This function returns the maximum number of channel available on the current
hardware/platform.
Fix the standalone build when using pulse.

The pulseaudio backend now needs to link agains dl because it uses dlopen and
friends.
Fix copy/paste error in src/cubeb_opensl.c
Merge pull request #25 from padenot/master

Max channel count + build fix