Respect --prefix

Currently, --prefix is not respected fully and it is impossible to install to a self-contained local prefix.
https://github.com/devttys0/binwalk/pull/51 solves this issue for the other libs, but the Python components are still installed in the system default path.

This passes --prefix to setup.py install to solve the issue.

autoreconf will have to be run. (Ideally, ./configure should be kept out of the repo via .gitignore and only provided in release tarballs.)