Updates tests to use the new nodeback API instead of single-result callbacks
Patches lib/tumblrwks.js to use the new nodeback API

To cope with users working on the old API, a transitional function
checkForDeprecatedCallbacks() is defined which should detect uses expecting the
older API and warn accordingly, as well as transition to the new API.
Updates examples in README.md to nodeback API
Merge pull request #4 from 00Davo/nodeback-api

Nodeback --- i.e., (err, res) callbacks --- API to address #3