Fix #1: Adding an option to print the level in String instead of Integer

This commit fixes Issue #1 by adding an option to display the level
using its String representation. Suggesting it to be the 0.2.0 version. This was first discussed at https://github.com/trentm/node-bunyan/issues/194, but since I use this module, I decided to start here... So contributing back!

*	modified:   lib/format-record.js
- Adding a method to map the level suggested by @olsonpm at
  https://github.com/trentm/node-bunyan/issues/194#issuecomment-64858117
- If the option is used, just change the level.

*	modified:   README.md
- Updating the documentation by adding the examples of outputing the
  reports using the levels in String.

*	new file:   example/bunyan-string-level.js
- Adding the new example that logs the level using the string
  representation.

*	modified:   package.json
- Bumping the version to 0.2.0