removing extra fs calls

nothing major, but i've removed a bunch of extra `fs.existsSync` and `fs.statSync` calls, which should speed things up a bit.  extra calls have been removed from:

- `cd`
- `ls`
- `cp`
- `rm`
- `mv`
Merge pull request #62 from stephenmathieson/extra-fs-calls

removing extra fs calls
Merge branch 'master' of github.com:arturadib/shelljs into ffi-exec-sync

Sync with shelljs master