RFP Creation page

> As a team member with RFP's enabled on my team

* Added the boolean column `teams.rfp_enabled`. Disabled by default. Update through rails console for now.

> When I am looking at an Issue in Bountysource
> And I want the issue resolved
> I click on the "Submit a Proposal!" button in the banner (#709) or the Proposals tab

Added the Proposals tab to issue show page. The tab is only visible if the issue tracker is owned by the team, and the team has the RFP feature enabled.

Without tab:
![image](https://cloud.githubusercontent.com/assets/692632/3095732/551a569e-e5cb-11e3-8af4-590fd4206f07.png)

With tab:
![image](https://cloud.githubusercontent.com/assets/692632/3095747/70d3895a-e5cb-11e3-9e1e-3a67344ced7f.png)

> and fill out any additional descriptions/budget/due date of the issue (see > https://github.com/bountysource/api/blob/master/app/models/request_for_proposal.rb annotations)

Added a form to create an RFP for the issue. 
![image](https://cloud.githubusercontent.com/assets/692632/3095764/9d2721ec-e5cb-11e3-9b1d-8e029b52f967.png)

> Then I click save & send
> And developers have a full description of the RFP to respond to

Render RFP on Proposals tab. Bonus: starting point for Proposals create form.
![image](https://cloud.githubusercontent.com/assets/692632/3095786/d899df1c-e5cb-11e3-8bb4-e7270c84708a.png)