Fixes #1 - Add xSince methods for time ranged queries

- Add ```xSince()``` methods in same style as existing lib methods. ```timestamp``` helper method takes reference from Algolia's own [hnsearch.js](https://github.com/algolia/hn-search/blob/master/app/assets/javascripts/hnsearch.js). Current limitation (besides option for before or between time range queries) is that only a single request is made, so the max number of hits returned is 1000. Could easily have more than that if  for example you do a ```getCommentsSince()``` with ```past_month``` param. So that will require multiple queries to get everything, but this may not even be desirable, as the number of hits can get huge. An issue can be opened for that. No tests added for the new methods.

- Updated README.md + minor fix (add missing methods)

- Set node true for JSHint - no false warnings now

- Addition to .gitignore