.vimrc and other updates

.vimrc:
- Changed default colorscheme to Lucius
- expand tabs into spaces
- set relative line numbers
- tab management mappings (\t* maps)

essay.tex template
- set headheight to 15.2 points, it looks better and works better with
the fancyhdr package
- added "\pagestyle{plain}" line

other:
- added Lucius colorscheme