Add roslz4 package; implements v1.4 of the lz4 streaming format

See:
http://fastcompression.blogspot.fr/2013/04/lz4-streaming-format-final.html
Use stream checksum in lz4 compression

The XXH32 hash of the uncompressed data stream is stored in the 4 bytes
immediately after the end of stream token.  Using it helps detect data
corruption errors and validates that decompression/compression did not
change anything.
Add python binding for roslz4
Implement lz4 compression in rosbag::Bag (C++)

- Use roslz4 for compression
- lz4_stream.cpp is a modified version of bz2_stream.cpp
- Streaming reading functions, startRead(), stopRead(), read() will be
  implemented later.
Implement lz4 compression in rosbag.Bag (python)
Add lz4 compression option in rosbag command line tools

- rosbag compress and record now take '--lz4' as an option
- Defaults are the same; rosbag compress uses bz2 and rosbag
  record uses nothing.
- The last flag determines the compression option
  (e.g., '--bz2 --lz4 --bz2' means bz2 will be used)