Fix windows style paths

- new method on `Connection`: `normalizePath`
- `Ftp`- and `SftpConnection` use `normalizePath` only when communicating with
server

Fixes: #22, #18, #17, #8

Justification to put fix in `Connection` instead of `FileManager`:
- `FileManager` could normalize the paths of the `Files` it emits, however the paths must still be `join`ed onto the Atom project directory, which will use `path.sep` anyway.
- `Connection#normalizePath` only needs to be called on paths sent to the server; local paths do not need to be normalized.