All partner (ie integrations) logos updated, including retina versions.

All partner logos are in, both in retina and normal resolutions. Screenshots below.

A few ideas:
1. should the group of 6 logos (single image with logos inline) above "view all our integrations" be a link back?
2. should we update page titles? Currently all page titles are "Integrations - GoCardless"
3. should I rearrange the order that the partners appear on the /partners/ page to match the 6 logos inline?
4. Are we happy with the sizes of the white logos in the hero? Xero in particular is pretty tiny. Are Sage and Freeagent appearing to be off-centre slightly?



![screen shot 2014-10-09 at 17 25 13](https://cloud.githubusercontent.com/assets/2903904/4580087/6e8a5a5e-4fd2-11e4-9007-0a55ab40cf3e.png)
![integrations gocardless](https://cloud.githubusercontent.com/assets/2903904/4580088/719f74b8-4fd2-11e4-938f-03b253e9cdd6.png)
![integrations gocardless2](https://cloud.githubusercontent.com/assets/2903904/4580092/7671856c-4fd2-11e4-953c-1b7c444cb4ac.png)
![gocardless for sage improve cashflow with direct debit gocardless](https://cloud.githubusercontent.com/assets/2903904/4580097/8135da66-4fd2-11e4-8f0c-fa6ade30e11e.png)
![integrations gocardless3](https://cloud.githubusercontent.com/assets/2903904/4580101/8bfa81cc-4fd2-11e4-8430-3dd2fec90d20.png)
![integrations gocardless4](https://cloud.githubusercontent.com/assets/2903904/4580104/8ede0abc-4fd2-11e4-92d3-4882e0ff139d.png)
![integrations gocardless5](https://cloud.githubusercontent.com/assets/2903904/4580105/9117f680-4fd2-11e4-9a07-2a319f0aba50.png)
Think this image is coming out a bit big, can we add `u-size5of6 u-center` to the containing div? Also, need to use JPG for this to keep file size down - only need to use PNGs for when we need transparency. Same change on all individual partner pages would be awesome, cheers! :+1: