Use an absolute path for DATADIR
Use an absolute path for DATADIR

Without this, I get a seg fault whenever I run the binary from a
directory other than gummi/src.

closes #369

turbopobre/gummi,bobi32/gummi,mlda065/gummi,mlda065/gummi,turbopobre/gummi,alexandervdm/gummi,turbopobre/gummi,bobi32/gummi,alexandervdm/gummi,mlda065/gummi,bobi32/gummi,alexandervdm/gummi,bobi32/gummi,mlda065/gummi,turbopobre/gummi