Retrying failed uploads and .maintenance file

[felixandersen]

Hello,

Two commits.

One that retries on network errors. Had some unstable connections to my servers that needed this.

The other modification is that a .maintenance file gets added when sync starts and removed once done. This is to put Wordpress-powered sites in maintenance mode preventing visitors reaching half-uploaded files. I realize that this might be something that can be configured on a per-remote basis. Let me know if this is of interest at all and I will make changes.