Added app search endpoint.

When polling the app list HTTP endpoint, you can now search the app list by
'id' or 'cmd' using the HTTP URL query format, like so:

  http://marathon-host:5500/v1/apps/search?id=some-app
Merge pull request #58 from brndnmtthws/app-list-filter

Added 'id' and 'cmd' query praams for app list.