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

tonyolag/gummi-vim,jmarshallnz/gummi,tonyolag/gummi-vim,jmarshallnz/gummi,tonyolag/gummi-vim,tonyolag/gummi-vim,jmarshallnz/gummi,jmarshallnz/gummi