The last commit fixes a huge memory leak where nassl was leaking an OpenSSL BIO on almost every connection.

In a simple script that runs 3 scan commands 20 times (https://gist.github.com/nabla-c0d3/fc633194f7a8509a7e68c931f1fbc471), memory usage at the end of the script went from 126856 kb to 50404 kb.
The memory was also relatively stable but was still increasing slightly on each of the 20 iterations. From my analysis, the biggest remaining leaks (which are much smaller than the big one I just fixed) are in OpenSSL 1.0.1e, mainly in SSL_do_handshake(); not much I can do about this.

This blog post helped me a lot, especially the section about umdh : https://benbernardblog.com/tracking-down-a-freaky-python-memory-leak/#usingumdhtosearchforcbasedmemoryleaks

The fixes have been released as part of nassl 1.0.2.