Add comments feature

Users can now comment on standups.
After your explanation:

i would probably suggest:

``` ruby
  it { is_expected.to permit(:text).for(:create, :params => { :comment => { :text => "Some text" }, :standup_id => 42 }) }
```

or:

``` ruby
  it { is_expected.to permit(:text).for(:create, 
                                                         :params => { :comment => { :text => "Some text" }, 
                                                         :standup_id => 42 }) }
```

This part is generally not covered in the ruby style guide, so it can be confusing. I think we use one of these styles above in Semaphore's code.