feat(server): Add a forwarding proxy for IE8.

* Proxy auth server requests when the url is prefixed with `/auth`
* Proxy oauth server requests when the url is prefixed with `/oauth`
* Show the user a `Service unavailable` error if the proxy request fails.

Other info:

* The oauth proxy tests are disabled until we have an OAuth server set up.

fixes #102
Disable the IE8 proxy in production.

* Add configuration items (fxaccount_proxy and oauth_proxy) to enable/disable the proxy.
* Add /config tests to ensure proxy URLs are returned for IE8
Add a TODO to enable OAuth tests with #1246
Merge pull request #1308 from mozilla/issue-102-proxy-auth-oauth-ie8

feat(server): Add a forwarding proxy for IE8.