Add Chrome SHA1 deprecation logic and unit tests. Details below:

1. Allow a platform to have no root store file, which will be treated as
   "trust all".

2. Add CryptoDeprecationPolicy to model any crypto deprecation.

3. Ubiquity score takes crypto deprecation in consideration.

4. Add a SHA1 deprecation warning to Bundle.Status.Message when
   necessary.
Merge pull request #27 from cloudflare/lziest/platform-crypto

Add Chrome SHA1 deprecation logic and unit tests. Details below: