Missing FileUtils aliases

The following FileUtils aliases are missing:

* remove -> rm
* rmtree -> rm_rf
* safe_unlink -> rm_f
* symlink -> ln_s
* copy -> cp
* move -> mv

There are a few more like `link -> ln` or `cmp -> compare_file`, but I didn't add them since those methods aren't implemented.

See [Ruby Doc](http://www.ruby-doc.org/stdlib-1.8.7/libdoc/fileutils/rdoc/FileUtils.html).