Creating remote db backup on db:up; keeping up to :keep_releases backups

See evolution/generator-evolve#1
Merge branch 'master' into 1-db-backup-keep-releases

Conflicts:
	lib/capistrano/tasks/db.rake
Added test for db backups
Merge pull request #3 from evolution/1-db-backup-keep-releases

Backup remote db on db:up, keeping up to N backups