fix remote_task inside a namespace

Previously defining `remote_task` inside a namespace resulted in
scoping the file name to the namespace. For example file named
`config/database.yml` inside `shared` namespace would result in:
`shared:config/database.yml`

That is surely unwanted behaviour. Fix is simple. Introducing
`Capistrano::UploadTask` that inherits from `Rake::FileCreationTask`.
These Rake File tasks are not carrying the namespace in their name.