add function makeDefaultConfig.

- To construct the argument that ngconf.default recevies from key-pair
  of envrionment variable/default value.
- uses the value of environment variable instead of the default value
  if it is given.
add test for environemnt variable configure.
add test for config default value assertion.
add test for s3 envrionment configuration.
use makeDefaultConfg to generate default settings of nconf.
Merge branch 'master' into feature/config-enc
use mock process.env to test config settings.
Merge pull request #149 from g0v/feature/config-enc

make all nconf options can be overwrite by environment variables.