Several changes

I saw that you included me into the repo, thanks for that!

I'm not sure if I can just push these changes to the master branch, though, so I'll stick to a regular PR first.

I made a few changes, normally nothing would change to the interface except for 
- people using those methods in asn1util that were unused in the project itself
- people using the encode() method, which they in fact should not

Currently I created a test to check if the existing ASN1 implementations generate correct DER encodings. I'm still looking for a way to test the bit string thing. Also, I'm including asn1lib in a part of Dartcoin where I need to parse DER encoding, so I'll see if it works there.