wrap remove_trailing_whitespace in augroup

You can read more about the motivation for this at
http://learnvimscriptthehardway.stevelosh.com/chapters/14.html#using-autocommands-in-your-vimrc
add command :WWS (Write With Spaces)

:WWS allows a file to be saved with trailing spaces, which are usually
removed automatially via a regular expression.

Fixes: #64
add ! to command assignment in .vimrc

without the ! in command!, the command assignment through an error when
re-applied. This was a problem when sourcing .vimrc in an already
running window.
Merge pull request #65 from salcode/removing-trailing-whitespace

Improvements to trailing whitespace handling