give better rspec messages by using default stubs
rework core detecting + refactor tests

  * avoid setting @cores if it's going to be returned anyway

  * handle exception when NUMBER_OF_PROCESSORS is empty string

  * handle *BSD group along with Dragonfly BSD

  * use IO.read + exception handling instead of File.exist?

  * group tests by os type

  * support using NUMBER_OF_PROCESSORS to override detection on EVERY os

  * test showing error (stderr)

  * use sysctl '-n' parameter to make things more robust

  * remove CELLULOID_IGNORE_CORE_COUNTING_ERRORS and error handling
Merge pull request #415 from chronomantic/rework_core_detecting

Rework core detecting