Support 64-bit platforms, also attempt to make .dll/.so/.dylib and unmount_cmd more platform indepdent.

@alexdu, not sure if you're still actively maintaining this project, but i'm trying to resurrect it for use with the book scanning project at https://github.com/diybookscanner/spreads.

These changes are really minor-- just getting it to be more platform independent.  I'm developing on on Mac OS X 10.9.2 (Mavericks), against gphoto2 version 2.5.3.1 (and python 2.7.6, in case that matters).  Fyi, liveview also works fine in this setup, running against pygame 1.9.1 (sdl-1.2.15).

I'm completely new to python (learning it to work with this project), so let me know if any of this is non-idiomatic.

Thanks!