PR related to #403 / Thread safety, actors

This contains the changes for https://github.com/vcr/vcr/issues/403 - particularly the use of celluloid to avoid concurrency problems. There remain some issues (3 failing specs under rbx/mri) - I spread FIXME-marks as reminders:

- exception handling within the vcr actor and signaling to the caller is not always working as intended
- the to_proc syntax in VCR.use_cassette(name, &req) is not yet supported
- celluloid raises the ruby requirements to 1.9.2+ - I didn't test the other supported platforms yet (e.g. jruby)

@myronmarston We could discuss/resolve the open questions in a remote session or asynchronously, just as you prefer.