Change shebang to use python3.
Handle missing colorsysplus module gracefully.
Config parsing: Allow homedir relative sound path.

This change allows this kind of sound specification:

[Sounds]
blip = ~/media/message.ogg
Find config file in user specific config location.

The previous code tried to look in $XDG_CONFIG_HOME, however this was
neither expanded, nor does this variable have to be set according to the
XDG spec. Using the appdirs module, we can find the correct place easily.
Merge pull request #1 from mheistermann/master

Various small improvements: python3 shebang, handle missing colorsysplus, homedir config parsing