Let's ask people to support us

Picking up from https://github.com/gittip/building.gittip.com/issues/78#issuecomment-51968337.

![screen shot 2014-08-12 at 3 57 20 pm](https://cloud.githubusercontent.com/assets/134455/3896827/f12bb966-225b-11e4-8231-f0af867f850e.png)

 - [x] only show message to people who are active users but aren't giving to Gittip
 - [x] style buttons a bit more
 - [x] wire up buttons&mdash;should immediately change the user's tip and replace the box with a "Thank you!" message and update the "Giving" amount in the header.
 - [x] close button should be deemphasized
 - [x] close button should set a bit in the db so we don't bug them further
 - [x] make the 7% statistic dynamic (`website.gsupport`?)
 - [x] adapt call to action to current percentage ("Will you join them?" over 80%, "Will you help us reach [xx%](https://github.com/gittip/www.gittip.com/pull/2629#issuecomment-51970979)?" below that)
 - [x] adapt the gift amounts to what the user gives/receives now (25¢/$2, 50¢/$5, $1/$10, $10/$100).
 - [x] make a page to explain what we've done recently and what we're working on next
 - [x] show the banner on the homepage
 - [x] make it responsive
 - [ ] prompt to add credit card if necessary
 - [ ] more confirmation on pricing page after clicking personal payment button