allow env.$rack_env.local to overwrite env.$rack_env
improve Integer parsing for Config#int

  docs state `The number or nil if the value couldn't be coerced to a Fixnum` but 'a'.to_i is 0.

  this patches fixes the behavior and return nil instead.
improve Time parsing in Config#time

  `require "time"` to allow more formats

  `rescue false` to match the docs behavior for returning `false`.
add test for env.$rack_env.local
add more tests for Config#int and Config#time
raise exception instead of silent error in Config#time and Config#int
Merge pull request #2 from lian/master

add env.$rack_env.local and two parsing fixes