add "match" support to SubjectItem Policy (fixes #44)

This change required an API change. SubjectItemPolicy was instantiated
like this in the past

R509::Config::SubjectItemPolicy.new("CN" => "required")

Now you'll need to do

R509::Config::SubjectItemPolicy.new("CN" => { :policy => "required" })

This is uglier in the common case, but allows this

R509::Config::SubjectItemPolicy.new("OU" => {
  :policy => "match",
  :value => "Engineering"
})

This commit also refactors the old config.rb into 3 separate files to
make it more readable.
refactor CRL Admin's #revoke_cert

-Errors if not passing nil or 0-6, 8-10
-Fixes #49, related to #48
add support for crl signing delegates, alternate crl digests, and

disable auto-generation of crls on R509::CRL::Administrator instantiation

Temporary tests have been written to accommodate these new features,
but the CRL::Administrator test suite should be rewritten to more
thoroughly test the class (shared examples, etc)
This completes 2 tasks from #48