first, rough cut at support for tags

This commit adds some initial support for adding tags by date range and for
querying by a single tag.

Here is an example of adding tags,

  POST /rhq-metrics/tags/numeric

  {
    "metric": "M1",
    "tags": ["test1", "test2", "test3"],
    "start": 1415108444636,
    "end": 1415744754582
  }

and an example of querying by tag,

  GET /rhq-metrics/tags/numeric/test3