Make ELB metrics plugin generic

Make the ELB metrics plugin fetch any ELB metric.  Defaults to Latency
to provide backwards compatibility with elb-latency-metrics.rb
Make age option an integer; use age in URL when querying API
Get rid of unneeded time comparison

* ?age= already does this for us
Get rid of unnecessary until
Time shouldn't be nil
fix suspicious critical threshold
Merge remote-tracking branch 'upstream/master'
The divisor for the average calculation should be based on the value array.
Merge pull request #278 from jkew/master

The divisor for the average calculation should be based on the value array
Merge pull request #276 from horus/suspicious_10

fix suspicious critical threshold (from 10 to 100)
Merge pull request #275 from skymob/add_age_to_check_aggregate

Make age option an integer; use age in URL when querying API
Merge pull request #274 from bashtoni/master

Make ELB metrics plugin generic