added URLGetRankedImageKeywords to php SDK
Changed Image keywordexample url
Added SDK support for imageKeywords Image API call, added jpg for Image API example call.
Added SDK support for imageKeywords Image API call, added jpg for Image API example call.
added tests for image api, taxonomy, image extraction, combined call.
Merge pull request #8 from redorb8/imageAPICalls

added URLGetRankedImageKeywords to php SDK