Add browability.py module
Add browsability.py module to exitmap.
This module attempts to fetch a web page via its URL, or IP address if URL failed. It also check the obtained content. It may result in :
- confirming the DNS misconfiguration of an exit: if URL failed but IP succeeded
- raising a problem of browsability with an exit : if URL and IP failed (DNS misconfiguration not excluded also)
- detecting other misconfiguration or altered content : if content is not as expected