Sponsor refactor

part of PT story https://www.pivotaltracker.com/story/show/79559336
- replaced id with osra num on sponsor index page
- added osra_num and affiliate to sponsor show page

![screenshot from 2014-09-26 19 50 24](https://cloud.githubusercontent.com/assets/7509180/4425869/1b996610-45ae-11e4-83f8-3e4981c8d10d.png)
![screenshot from 2014-09-26 19 51 00](https://cloud.githubusercontent.com/assets/7509180/4425876/22cd2872-45ae-11e4-9d7e-d5e7653f7020.png)

What do you think of the following structure for the sake of readability?

```ruby
  describe 'branch or organization affiliation' do
    describe 'must be affiliated to 1 branch or 1 organization' do
      subject(:sponsor) { build_stubbed :sponsor }
      let(:organization) { build_stubbed :organization }
      let(:branch) { build_stubbed :branch }

      it 'should not be valid when not affiliated with either branch or organization' do
        sponsor.organization, sponsor.branch = nil
        expect(sponsor).not_to be_valid
      end

      it 'should not be valid when affiliated with both branch and organization' do
        sponsor.branch = branch
        sponsor.organization = organization
        expect(sponsor).not_to be_valid
      end

      context 'when affiliated with a branch but not an organization' do
        before do
          sponsor.branch = branch
          sponsor.organization = nil
        end

        it { is_expected.to be_valid }
        it 'returns branch name as affiliate' do
          expect(sponsor.affiliate).to eq branch.name
        end
      end

      context 'when affiliated with an organization but not a branch' do
        before do
          sponsor.branch = nil
          sponsor.organization = organization
        end

        it { is_expected.to be_valid }
        it 'returns branch name as affiliate' do
          expect(sponsor.affiliate).to eq organization.name
        end
      end
    end
  end
```