Although Github reserves the 'visionmedia' user space(a visit to there
will be redirected to 'tj'), it's better to
rename the links from visionmedia/git-extras to tj/git-extras
`git squash` refactor:
- first argument can be a branch or a commit reference
- the `--me` argument is not required anymore
- use `git reset --soft` is squashing on a current branch
- if squashing on master branch, prompt for confirmation
- does not delete provided branch after squashing
- fix bug: if `git merge --squash` command fails, exit script
Update README and man pages.
Fixes#95Closes#198
