Added SiLK file writer for exporting from bro log to SiLK
Updated core to current Bro logs. Added SiLK.

- Adjusted AsciiLogSpec to skip the timestamp. TODO add this to BroLogFile
- Updated BroLogManager and BroLogUtil to handle new filenames
- Added SilkFileExporter. Converts conn log to SiLK binary flow archive.
  TODO: Fix behavior of skipping flows that are too large for SiLK.