Users can create pitches for journalists

Trello: https://trello.com/c/0oul2GrI/44-users-create-pitches

In this feature, users can create write one pitch, assign multiple journalists and have an individual pitch created for each journalist.

I suspect the batch_pitch_params part of the batch_pitch controller can be refactored.

Includes addition of pry & better_errors gems
likely want to constrain the status to be part of a list. Something like:

``` ruby
STATUSES = ["Not Sent", "Sent", "Declined", "Awesomesauce"]
validates :inclusion_of, :status, in: STATUSES
```