Add GemRun and GemVersion classes

GemRun can load and run gems within Ruby code.  It is now
used in the metric bin/mf-* executables

GemVersion uses RubyGems to read the metric_fu.gemspec and can
return the specified version for a given metric gem
Remove metric_fu_requires
Capture metric output when executing
Consolidate code that runs external metrics to the generator
Capture all output without shelling out; use Open3.popen3
Include backtrace in system exit
Put gem summary debug in GemRun
Merge pull request #157 from metricfu/no_shelling_out

[Feature] Run external metrics without shelling out