Added docstrings to configuration-related modules

Ignore-this: 2d9a3520aeec02eb1e1e81db796bd98

darcs-hash:20090317133122-1e3d8-90fef6207a0d93c930dac1456c59871c943dc0b5
Changed default value of user_agent setting

Ignore-this: 475e2f2dd9d0787791fe874b5845afaf

darcs-hash:20090317161452-1e3d8-61c25e1613ceb496fdff9fcfc0e90fabd591df24