Add roslz4 package

Contains C++ and python libraries for v1.4 of the lz4 streaming format.
Both libraries use the system version of lz4 and use default values for
the stream parameters (e.g., includes a stream checksum, but not block
checksums).  See:
http://fastcompression.blogspot.fr/2013/04/lz4-streaming-format-final.html

The C++ library provide a stream style API for
compressing/decompressing data, similar to the bzip library.  They also
contain convenience buffer to buffer compression/decompression.

The python library provides an API similar to the bz2 module and works
with python 2 and 3.
Add lz4 compression to rosbag (python and C++)
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.

"rosbag compress" and "rosbag record", implemented by rosbag_main.py, use
the last flag to determine the compression option (e.g., '--bz2 --lz4
--bz2' means bz2 will be used).  This choice is consistent with other
python commands (e.g., "rosbag compress --output-dir=/foo
--output-dir=/tmp file.bag" will put the compressed bag in /tmp).

In contrast, the C++ record executable will raise an error if it is
passed multiple compression types.  This is consistent with boost's
program_options which by default forbids multiple occurrences of the
same flag.
Merge pull request #356 from bcharrow/lz4

lz4 for rosbag