Users can review announcements & pitches independently of each other

Trello: https://trello.com/c/Kpu0RH61/60-users-can-view-pitches-announcements

NOTES: 
Apologies for the size of the PR, I wanted this refactoring to leave the user able to do everything they were previously able to do (create announcements & pitches).

I want users to have the freedom to view pitches and announcements independently of each other. This refactoring enables to the user to view all pitches or all announcements belonging to them. This results in indexes sometimes displaying all data OR filtered data depending on which route they're coming through. 

One challenge this direction presents is forcing the user to specify extra information in forms. If a user wants to create a new pitch and comes from /pitches, it makes sense for them to specify which client announcement the pitch is for. However, if the user is coming from announcements/3/pitches, I'd (at the very least) like for the drop-down menu to be pre-set to the announcement name. I will look to do this (along with a few other refactorings) in a future PR.

![viewing](https://cloud.githubusercontent.com/assets/4560985/5268293/30b13d0a-7a26-11e4-9a39-cc7136e4c6c2.gif)


Rather than merging in the user_id in the `announcement_params` method, could you build the announcement through the `current_user` to get the association that way?