added getExtentBuffer and getTileUrls to offlineTileEnabler
updated README
fixed minor bugs in new helper methods
tweaked and renamed getTileUrls to getTileUrlsByExtent to prevent confusion with getTileUrl override
updated spec to include two new helper methods
updated readme to reflect change of getTileUrlsByExtent name
validate if urls are actual urls
removed getTileUrl() call
added assert statement to detect if img is undefined
Merge pull request #152 from andygup/new_methods

Added two helper methods to offlineTileEnabler