Update up function, add 2 shorthand functions
Update the 'up' function; now uses only 1 cd call

Previously up issued one cd command for each level the user specified to go
up. Now it builds a string corresponding to the number of levels and then
calls cd once. This makes it possible to issue `cd -` to go to the previous
directory.
Rename install script to dotfiles
Update $PS1 variable using faster git parsing code

Thanks to @paulirish for his dotfiles