Set current working directory when running as PyInstaller bundle.

This helps finding the DOSVOX executables when they are on the same directory as xmppvox.exe.

Note that DOSVOX might run "xmppvox.exe" having the current working directory set to something like "c:\winvox\treino".
Compare executable name case-insensitively.

For Windows, xmppvox.exe, Xmppvox.exe and XMPPVOX.exe are all equally good. Renaming xmppvox.exe to XMPPVOX.EXE would not work.
Silent keyboard interrupts.

No more dirty tracebacks when I finish execution with CTRL-C.
Add missing return statement.
Add help strings for project's online presence.

Tell users about website and mailing-list.

Closes #3.
Reformat lines.

Everything fits in one line, no need to two poorly indented lines.