[#2] OCSP request & response parsing

Preparations to start moving towards getting rid of DigiDocService.

[DigiDoc's file format standard](https://www.sk.ee/repository/bdoc-spec21.pdf) has a few gotchas regarding OCSP. It requires sending hashes of signatures as part of an OCSP request as [nonces](http://www.alvestrand.no/objectid/1.3.6.1.5.5.7.48.1.2.html).

Parsing responses using ASN.1 removes the need to do any hacky parsing of openSSL's CLI output.

A few things should be done before this can be merged:

* [x] move ASN.1 declarations to separate classes (ideally `Asn1\Ocsp` and `Asn1\X509`, the latter would mostly wrap `phpseclib`'s `X509` class);
* [ ] ~~consider completely skipping openSSL for making requests as there's no way to set custom nonces (probably separate PR)~~ won't do at this point;
* [ ] ~~improve error checking in `Response` now that we can do all sorts of fancy stuff with the response~~ (#5);