Allow CPC to run from anywhere.

This construction allows CPC to reference the candlepin_api.rb file
without depending on the working directory.  It is compatible with Ruby
1.8.x, 1.9.x, and 2.0.  See http://stackoverflow.com/questions/4325759
Merge pull request #486 from candlepin/awood/cpc_require_relative

Allow CPC to run from anywhere.