Added functionality for pre and post samples to burst tagger

Hi,

I have made some modifications to the existing burst tagger in the GNU
Radio source code.
It is now able to specify an amount of pre and post samples, which are
additionally tagged before and after the tag signal.

I have put my changes on my GitHub account, which is:
https://[email protected]/bartel/gnuradio-bartel.git

Since this is my first work with Git and my first direct change to the
GNU Radio source code, I would be glad for any advise on improving my
code/changes.

I have also tested my code locally, since I am not quite sure, how to
add a QA code for a C++ block to GNU Radio. Should it be with cppunit or
pyunit?
Thank’s for helping.

Best regards,
Daniel