Added AppAlerts latest alert to the status feed for both authenticated and anonymous users. When there are no alerts in the calcentral-alerts/feed, the alert element will be absent

Tasks Included
- fixture: added the fake xml for the app alerts feature in public/dummy/app_alerts.xml
- config: added app_alerts_proxy settings to main config file including a feature flag, a 2 minute expiration time, a fake mode option, and an api url
- confing: turned on the app_alerts feature for development

Create AppAlertProxy:
- Format timestamps using DatedFeed
- Use get_latest method to get the most recent alert (cached as "user/global-alert/AppAlertsProxy" for 2 minutes)
- Use get method to get all messages (cache key: "user/global-alerts/AppAlertsProxy")
  !! This method isn't currently may never be used. Pending definitions.
- Include camelCaseless entry example in dummy/status.json:

    "alert": {
      "title": "CalCentral Scheduled Maintenance 3/17",
      "teaser": "CalCentral v32 release at 6 - 7am",
      "url": "http://ets-dev.berkeley.edu/calcentral-scheduled-maintenance-317",
      "timestamp": {
        "epoch": 1393495946,
        "date_time": "2014-02-27T02:12:26-08:00",
        "date_string": "2/27"
      }
    }

* At the momement, the AppAlertyProxy raises no specific exceptions.  It simply rescues them and logs them as errors. Result for AppAlertProxy.get_latest are either nil present.

? Question: I'm unsure if we can rely on the feed to keep returning mutliple items in descending order by <PostDate>.  If we can, then we don't have to sort, so I've kept that commented until I know the answer.

AppAlertsProxy: spec/lib/proxies/app_alerts_proxy_spec.rb
Added tests for AppAlertsProxy trying to cover
- failures
  - unfound fake feed
  - unfound real feed
  - parsing invalid xml
  - parsing unexpected xml
- sucesses
  - find multiple messages -> aap_instance.get
  - find latest message    -> aap_instance.get_latest
- cache writing

? Question: for @chris.  If I'm going to continue to shut up errors this way, I need to fix my tests that are not correctly detecting the messages writtent to the log files.

Added tests to UserApiController for including an alert (or not) in the status feed.

Amendments based on Chris's line notes on the PR:
1. configuration of the fake parameter
   app_alerts_proxy.fake settings were wrong according to Chris's line note:
   Fake should default to false here. Default it to true in development.yml and test.yml, but not in testext.yml.
   The features.app_alerts value is true in dev and test, but not in settings.yml.  I think that's right.

2. app_alerts_proxy_spec
   Renamed real_proxy to failing_proxy because it's doomed to fail.
   Also removed test of the proxy.get method, which Chris has asked me to remove

3. Don't check feature flags in specs
   The feature.app_alerts setting is set to true in test.yml and in testext.yml, too, so there's no need to check the feature flag in a spec.  Thanks Chris!

4. remove unused AppAlertProxy::get method
   Since the get method isn't used anywhere yet, I removed it.
   Chris says we can always revive it later if we need it.

4. Keep dummy xml feeds in fixtures/xml instead of in public/dummy. Requested by Chris.
Merge pull request #2213 from chikoon/CLC-3248-backend-for-user-alerts

CLC-3248-backend-for-user-alerts