Add output="mws-ids" to MWS Query.

Summary:
 - abstract mws response output formatters.
 - closes #43.

Test Plan:
./bin/mwsd -i data/zbl
./scripts/send-qvar-query.sh 9090 0 0
# edit scripts/send-qvar-query.sh and add output="mws-ids"
./scripts/send-qvar-query.sh 9090 0 0
./scripts/send-qvar-query.sh 9090 0 100

Reviewers: radu.hambasan, m.iancu

Reviewed By: radu.hambasan

Differential Revision: http://phab.code4fun.de/D72