add support for multiple sensor queries

[satta]

This PR addresses #35, allowing a query for more than one sensor ID in the entries() query. This is backwards compatible, since

query {
  entries(rdata:"1.2.3.4", sensor_id:["foo"]) {
    rrname
    sensor_id
  }
}

seems to be equivalent to

query {
  entries(rdata:"1.2.3.4", sensor_id:"foo") {
    rrname
    sensor_id
  }
}

which matches the previously used syntax.

[magenbluten]

maybe we should somewhere document that the limit is per sensor, i.e. if you query for 5 sensors with a limit of 100 you could get up to 500 results.

[satta]

maybe we should somewhere document that the limit is per sensor, i.e. if you query for 5 sensors with a limit of 100 you could get up to 500 results.

Well, it's not per sensor at the moment! The implementation in this PR treats the limit parameter as describing the sum of all results.

[satta]

Do we want to merge this or not? Any comments? Suggestions? Would it be more intuitive (@petromoldovan?) if we had a limit per sensor?