Added libmemcached in Mac OSX section & troubleshooting.rst file changed

I tried to install/build open comparison  without `libmemcached` package and get following error message.

'''In file included from _pylibmcmodule.c:34:

./_pylibmcmodule.h:42:10: fatal error: 'libmemcached/memcached.h' file not found

#include <libmemcached/memcached.h>

         ^

1 error generated.

error: command 'cc' failed with exit status 1''' 

then I installed `libmemcached` using brew. and it works.