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

[marcellodesales]

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

[thlorenz]

Thanks, published as 0.2