Issue: SSHKit::Backend::Netssh binding change in run method breaks lookup in 'normal' block scope

Hi,

in `SSHKit::Backend::Netssh#run` instance_exec is used to change the binding of the given block. With this binding change it is difficult to encapsulate an action in a service object for instance you have a cap-task:

``` ruby
desc 'Setup nginx configuration'
task :setup do
Nginx.new(self).setup
end
```

``` ruby
class Nginx < SimpleDelegator
...
def upload_files
on roles nginx_roles do |host|
set :host_server_name, host.properties.fetch(:host_server_name)
template 'assets.host.site', tmp_config_file
end
end

def tmp_config_file
'remote_file_name'
end
...
end
```

This does not work because the block is executed in the context of the `SSHKit::Backend::Netssh` object context and does not know the method tmp_config_file.

Why do I want to do such things? Testing! Rake and Cap tasks are difficult to test.
When I encapsulate the actions I can test the a little.

regards
dieter