switched .recommendation and .similarity from GET to POST

items.recommendation was returning an empty object. Switching from .post(endpoint) to  .get(endpoint) on line 122 fixed this. Also fixed this for item.similarity. It was experiencing the same issue, however, I am not running a similarity engine, so I did not get a chance to test that fix.
Merge pull request #6 from the8thbit/master

switched .recommendation and .similarity from GET to POST