got rid of dependency on posix module; replaced posix_getpid with getmypid
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 commit '8e42b9ceacec8af7ec16b965cada9d320fd93ff1'

Improves documentation.