Fix windows style paths

[jordanbtucker]

• 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 joined 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.

[coveralls]

Coverage increased (+0.07%) when pulling 56cd148 on jordanbtucker:fix-win-paths into c62107a on amoussard:master.