Cert loading from PEM & restructuring

- Added cert loading

- Extracted X509 tests

- Extracted `hash_str_to_vec` function into `utils.rs` as it is used
  in a couple of tests now

- Fixed `hash_str_to_vec`, funny enough prev test with it was working
  because I’ve made another mistake by providing VerifyNone instead of
  VerifyPeer and those 2 mistakes compensated each other. Since
  `hash_str_to_vec` is private there was no errors on client code
You can use http://doc.rust-lang.org/std/io/util/struct.NullWriter.html to avoid having to open a FileStream.