support 64 bit fd's
use six's integer_types instead of py3 incompatible type 'long'
allow tests to run even without execinfo.h
Merge pull request #20 from kouk/longint

Add support for file descriptors represented as `long` on Python 2.x.  This fixes many failing tests on 64 bit Windows w/ 64 bit Python.

Also some other changes to make `memdbg` optional (it is pretty glibc-specific).