Installer tweaks

Instructions for compiling from source, also using Slackware build scripts for those of us in that boat (I can, after all, confirm that it works).  Added alternate torrent file (with gpg sig) from bitcoin.org into extra/ in case some end user wants to use it for whatever reason.

Added PIP to the installation sequence - it ought to be added at least to the Windows and OS X instructions, even if only to make sure those users have setuptools installed.

Adjusted setup.py to try setuptools and only if that fails go to distutils.core.