Only install readline on systems that need it

The readline module is part of the standard Python package found in most common Linux distributions (Debian/Ubuntu/Mint/Fedora/etc.). The only major Posix distribution requiring an explicit readline installation is Mac OS X. Another notable exception is the ActivePython distribution which also ships without readline.
Merge pull request #34 from ludwigschwardt/patch-1

Only install readline on systems that need it