Check if remote file changed since last push

See #50 : Before uploading a file, it checks if the file was modified on the server since the last deployment. If yes, it asks interactively:
- Overrwrite: upload anyway (as it did before)
- Download to working copy: do not upload, instead the file of the server is downloaded to the working copy so that differences can be treated (git diff) (Rem. if you do git checkout --, the file will not be uploaded the next time)
- Never upload this file (add it to .git-ftp-ignore, so that it won't be asked the next time)

Currently only does the 'git ftp push' part of it. 

Maybe there should be a command line option to allow non-interactive use.