jar loading refactor

this is intended as a fix for the log4j jar loading issue in https://logstash.jira.com/browse/LOGSTASH-2055 

since there were many places where this kind of code was present, I refactored the jar loading into the `logstash/environment.rb` class. I added related plugins `register` specs. This DRYs up the code and adds robustness & better exceptions/error handling.

Also, somewhat related:
- removed `logstash/test.rb` and `bin/logstash test` command
- while in this area, made `bin/logstash rspec` to run all specs by default
- noticed that `make test` was missing deep specs