Handle multiline messages

Addresses #30 
@fandrei,

The technique used by this test - start 2 nxlog instances, shipper => syslog://127.0.0.1 => reciever might be applicable for some of the integration tests.

Ie, rather than shipping to meta.logsearch.io, you just ship between 2 nxlog instances, and check that what you received is what you expected to be sent.

I still think we need some integration tests that test all the way to meta.logsearch.io; but the rest could be implemented is this style - faster, and doesn't depend on meta.logsearch.io being available / correct.