Initial cut at making the manifest commands async

More info for @Inecas and @komidore64. Related to 

https://github.com/Katello/hammer-cli-katello/pull/49

When I run this I get:

```
NoMethodError (undefined method `[]' for nil:NilClass):
    /home/bkearney/.rvm/gems/ruby-1.9.3-p484/bundler/gems/hammer-cli-foreman-tasks-ff213ae3e73e/lib/hammer_cli_foreman_tasks/task_progress.rb:54:in `task_pending?'
    /home/bkearney/.rvm/gems/ruby-1.9.3-p484/bundler/gems/hammer-cli-foreman-tasks-ff213ae3e73e/lib/hammer_cli_foreman_tasks/task_progress.rb:14:in `render'
    /home/bkearney/.rvm/gems/ruby-1.9.3-p484/bundler/gems/hammer-cli-foreman-tasks-ff213ae3e73e/lib/hammer_cli_foreman_tasks/helper.rb:7:in `block in task_progress'
    /home/bkearney/.rvm/gems/ruby-1.9.3-p484/bundler/gems/hammer-cli-foreman-tasks-ff213ae3e73e/lib/hammer_cli_foreman_tasks/helper.rb:6:in `tap'
    /home/bkearney/.rvm/gems/ruby-1.9.3-p484/bundler/gems/hammer-cli-foreman-tasks-ff213ae3e73e/lib/hammer_cli_foreman_tasks/helper.rb:6:in `task_progress'
    /home/bkearney/.rvm/gems/ruby-1.9.3-p484/bundler/gems/hammer-cli-foreman-tasks-ff213ae3e73e/lib/hammer_cli_foreman_tasks/async_command.rb:11:in `execute'
    /home/bkearney/.rvm/gems/ruby-1.9.3-p484/gems/clamp-0.6.3/lib/clamp/command.rb:67:in `run'
    /home/bkearney/code/hammer-cli/lib/hammer_cli/abstract.rb:22:in `run'
    /home/bkearney/.rvm/gems/ruby-1.9.3-p484/gems/clamp-0.6.3/lib/clamp/subcommand/execution.rb:11:in `execute'
    /home/bkearney/.rvm/gems/ruby-1.9.3-p484/gems/clamp-0.6.3/lib/clamp/command.rb:67:in `run'
    /home/bkearney/code/hammer-cli/lib/hammer_cli/abstract.rb:22:in `run'
    /home/bkearney/.rvm/gems/ruby-1.9.3-p484/gems/clamp-0.6.3/lib/clamp/subcommand/execution.rb:11:in `execute'
    /home/bkearney/.rvm/gems/ruby-1.9.3-p484/gems/clamp-0.6.3/lib/clamp/command.rb:67:in `run'
    /home/bkearney/code/hammer-cli/lib/hammer_cli/abstract.rb:22:in `run'
    /home/bkearney/.rvm/gems/ruby-1.9.3-p484/gems/clamp-0.6.3/lib/clamp/command.rb:125:in `run'
    bin/hammer:77:in `<main>'
```