api: Add an /api/ping path

A GET request to /api/ping returns a 200 response with no content body,
and the x-pip-db-records header set to the number of records in the
database. For example:

    GET /api/ping HTTP/1.1
    Accept: */*

    HTTP/1.1 200 OK
    Content-Length: 0
    Date: Mon, 07 Apr 2014 19:41:25 GMT
    Server: Jetty(7.6.1.v20120215)
    x-pip-db-records: 5773

Issue #334.
Merge branch 'issue/334'