Use more formal/complete HSTS parser

Ideally, there would already be a good tested Python library for doing this.

For reference, here's the parser I wrote in Ruby (with careful attention to the HSTS RFC):
https://github.com/benbalter/site-inspector/blob/erics-mode/lib/site-inspector.rb#L28-L84

Fixing this issue should include unit tests validating that the parser meets the HSTS spec. An example of that in a previous (Ruby) project can be seen here:

https://github.com/benbalter/site-inspector/blob/erics-mode/test/test_hsts.rb