Added Image File Object

Added Image File Object, an extension of the File Object, for capturing general properties of image files (e.g. JPEG, GIF, etc.). 

Let me know if the annotation for Compression_Algorithm makes sense; it seems that for many image file formats the compression algorithm is an inherent part of the format (e.g. GIF), while in others it can be configured (e.g. TIFF) - I tried to highlight this. Also, I added an "image_is_compressed" attribute for capturing whether the image in the file is compressed or not; this was the only discrepancy from the original issue.

This should close #117.