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.yaml 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.