Write a page for new contributors

In conversation with @ceboudreaux in
[IRC](https://botbot.me/freenode/gittip/msg/4258008/) and @heidigardner
on #1120, it came out that Gittip has many sources of information and
it's hard to get oriented as a new contributor. Here's a page that aims
to fix this problem.
Learn to count
Rework first paragraph of /for/contributors/

Per feedback from @heidigardner and @ceboudreaux.

 - "new kind of company" -> "new kind of organization"
 - change "share" to "take" to indicate method of distribution
 - add two links and move a third
Specify that it's the *email* newsletter
Update balanced account links for new dashboard

Balanced has a new dashboard now, and we should use it (it has a
different URL).
Add departments and call to action
new sign in button
Merge branch 'master' of github.com:jeromegn/www.gittip.com into sign-in-with
oh, had created all that junk trying to watch files with sass
more junk outstanding
Note ShareTribe on README
Tighten up opening and closing

Per @cakey in [IRC](https://botbot.me/freenode/gittip/msg/4555032/).
remove border on all buttons, fix bank and credit pages
Merge branch 'master' into sign-in-with
Smile!
buttons, tests, profit
fixing the dropdown and join button
Be less formal

Per @cakey in [IRC](https://botbot.me/freenode/gittip/msg/4555267/)
flash button, 80 char limit, remove commented code
also, don't flash on hover