Clean up task nice value calculation

Also, change the formula to `log(priority) * (due - now)`. This could
use some fiddling, though.
Modernize Makefile a bit

This includes a new install target, which will install to /usr/bin.

Close #9
Compose a manpage

The Makefile will install the manpage to /usr/share/man/man1 when
appropriate.

Ref #10
Fix case-sensitivity in `done` command

Close #18
Version 0.2.2