updated README.md
added .vimrc

The .vimrc is based on
https://github.com/mathiasbynens/dotfiles/blob/master/.vimrc, which I
just copied (and removed the line about the mouse support, since that
does not seem to work on SL6).
added .bashrc
added .gitconfig and .gitignore

.gitignore contains mostly LaTeX stuff, since most of my *dirty* repos
 are LaTeXy.
.gitconfig mostly has some aliases in it which I think are useful.
Additionally, it sets a global .gitconfig, which resides in my home
directory (which is currently a hard link to this file).
added .screenrc