refactor(pkix): remove pem.Block from member vars

The object has no need to keep PEM-format representaion, because
this could be generated by itself.
feat(pkix/csr): Add certificate signing request
feat(pkix/cert): add creation of certificate for host
chore: add github.com/jstemmer/pkcs10
chore(pkix): add some comments
Merge pull request #6 from unihorn/31

feat(pkix): add certificate signing request and certificate for host