PERL-267 memory leak fixes. plus, making sure libson is linked when buil...

PERL-267
https://jira.mongodb.org/browse/PERL-267

This at least fixes some of the memory leaks. When doing and endless loop of inserts, I saw no memory growth after this. There was a couple of leaks in setting up a connection, and in the cursor. I also fixed up the Makefile.PL to make sure that libson was linked in. 

I hope this at least helps.