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
fix(tests): Make IE10/IE11 pass all the mocha tests.

* IE10 passes with no additional work.
* IE11 requires mouse interaction for the tests to complete correctly.

issue #1334
Add a TODO to enable OAuth tests with #1246
Only gather entropy from the server if the browser does not support crypto.getRandomValues.
Merge pull request #1308 from mozilla/issue-102-proxy-auth-oauth-ie8

feat(server): Add a forwarding proxy for IE8.
Merge pull request #1355 from mozilla/issue-1334-ie-test-failures

fix(tests): Make IE10/IE11 pass all the mocha tests.
feat(views): add custom event WebChannel, add NullChannel