Add fields of all SD-ELEMENTS, not just the first.

Before this only the fields of the first RFC5424 SD-ELEMENT have been
added to the fields map. This is fixed now by iterating over all
existing SD-ELEMENTS returned by the syslog parser.

Known problem:

If two different SD-ELEMENTs share the same key in the SD-PARAMS, they
overwrite each other. See code comments for details.
Merge branch 'master' into syslog-multi-sd-elements
Add config setting for expanding SD-ELEMENTs in SyslogCodec

Activating "expand_structured_data" will now prefix the extracted attributes
from structured syslog messages with the SD-ID of the SD-ELEMENT they've been
extracted from, e. g. '[meta key="val"][mySD@12345 key="val"]' would result in
two attributes "meta_key=val" and "mySD@12345_key=val" in the final GELF message.
Replace inner static class SyslogCodec.StructuredSyslog with instance method
Remove wrong initial size from HashMap allocation in SyslogCodec
Remove duplicate license header
Merge pull request #744 from Graylog2/syslog-multi-sd-elements

Add fields of all SD-ELEMENTS, not just the first.