Initial commit to fix #175 accompained by test cases.

This commit adds the function that initially fixes #175 by disabling the
record being emitted. Requirements are described in the issue #175.

*	modified:   lib/bunyan.js
- Adding a helper method to disable objects by properties

*	new file:   test/log-env-disabled.test.js
- Adding the disable test case by setting the environment var before loading
  the record.