Fix reading window title from _NET_WM_NAME.

XGetWindowProperty() returns 0 on success.

Closes #236
Add an option to display a tray icon.

Also implement "Minimize to tray" functionality.
Use specific monospace fonts as fallback on Mac OS X.

Qt (4.8.6) doesn't seem to be able to resolve the generic monospace
font family.

Closes #214
Coding style fixes.
Add Travis CI config.