Boolean AND as standard search operator

See https://wiki1.hbz-nrw.de/display/SEM/NWBib-Treffen+10.9.2014

- Implicitly use boolean AND for plain queries over all fields (this pull request)
- Use tweaked API with Operator.AND for specific field queries (see https://github.com/hbz/lobid/pull/58)

@acka47 assigning to you, we don't have an issue for this. If you +1 I will merge and deploy to production.

Deployed to staging:

http://test.lobid.org/nwbib/search?query=glessen+mühlen (all fields)
http://test.lobid.org/nwbib/search?name=paderborn+fußball (title)
http://test.lobid.org/nwbib/search?publisher=gruner+jahr (publisher)