Repeater input consistency & system backups

Happy to add some new features and minor fixes:

#39 - now the repeater inputs will have consistent order based on how they are displayed in the CMS
#42 - system.db, analytics.db and uploads can be backed up using http endpoint: /admin/backup and providing a query param source set to {system,analytics,uploads}. See the comment explaining usage: #42 (comment) (will be documented in the wiki)

To get it, run:

$ go1.8rc2 get -u github.com/ponzu-cms/ponzu/... # sub go1.8rc2 with your Go binary if you run 1.8
$ cd $GOPATH/src/your/project && ponzu upgrade