add extract thumbnail image from exif data functionality 

I needed to extract thumbnails from exif data so i implemented that function and some other improvements, so if anyone interested. i don't know if it should be a pull request, if not, just ignore it
details of my changes:

1. use strict added to prevent global space pollution
2. added comments to some code parts
3. slicing only first 128KB of the original image file - this way the initial ViewData buffer being created much faster! exif data should be always less then 64kbX2
4. log masseges are now handled by local function
5. refactoring variable names within some functions
6. exif thumbnail image IFD tags added
7. the thumbnail data (if found) will be extracted into exifdata.thumbnail property
8. file indentation changes - tabs replaced with "2 spaces"