Remove mappings for :bprevious and :bnext

Unimpaired already provides mappings for this and more.
Mappings for opening and sourcing my vimrc

Taken from chapter 7 of [Learn Vimscript the Hard Way]
(http://learnvimscriptthehardway.stevelosh.com/).
Remove whitespace-removing autocommand
Mapping for cleaning trailing whitespace
Wrap autocommands in a group, clear it at start up

Learned in chapter 14 of LVSTHW.