Make parse_many return a Result<Vec<Pem>>

[mhils]

Thanks for the prompt reply in #27! 🍰

This PR changes parse_many to return a Result. Feel free to discard the CHANGELOG.md if you do not want to maintain it, I figured it'd be somewhat useful with the breaking change. 😃

[jcreekmore]

You have some test failures:

---- src/lib.rs - parse_many (line 289) stdout ----
error[E0599]: no method named `len` found for enum `std::result::Result<Vec<Pem>, PemError>` in the current scope
  --> src/lib.rs:316:18
   |
30 |  assert_eq!(pems.len(), 2);
   |                  ^^^ method not found in `std::result::Result<Vec<Pem>, PemError>`

error[E0608]: cannot index into a value of type `std::result::Result<Vec<Pem>, PemError>`
  --> src/lib.rs:317:13
   |
31 |  assert_eq!(pems[0].tag, "INTERMEDIATE CERT");
   |             ^^^^^^^

error[E0608]: cannot index into a value of type `std::result::Result<Vec<Pem>, PemError>`
  --> src/lib.rs:318:13
   |
32 |  assert_eq!(pems[1].tag, "CERTIFICATE");
   |             ^^^^^^^

error: aborting due to 3 previous errors

[mhils]

I must have missed the doctests somehow, apologies. I've pushed a fix. :)