Add "id" search for feature parity with V1 API

The V1 API "search" endpoint allows searching both on "cmd" and on "id".
V2 already supports "cmd", and so this adds "id" to make V2 on par with
V1's functionality.

Example:

    curl -H "Accept: application/json" localhost:8080/v2/apps?id=hadoop_
Refactor `search` to be more readable per feedback
Merge pull request #216 from mesosphere/v1-v2-search-parity

Add "id" search for feature parity with V1 API