Minimal EXIF support for DNG and CR2

[art-den]

Hi, I did minimal EXIF support for DNG and CR2 RAW files. First, I did too mach changes in tiff.rs. I'm not sure I didn't break something.

1. Fixed: TiffIFD::find_ifds_with_tag returned same &TiffIFD twice
2. Added constants T_* for tag types
3. Added single functions get_***_entry_val into tiff.rs to convert raw bytes of TiffEntry to values. Functions are used both from tiff.rs and exif.rs
4. Fixed: getting of rational values worked wrong if integral values were too large (u32 -> f32 conversion loses data for large u32)
5. Support of FLOAT and DOUBLE tag types
6. The TiffEntry::get_u32 algorithm has been changed.
7. New module exif.rs. Contains trait for getting of Exif data + implementation that use get_***_entry_val functions to return values. Only tags listed in NativeExifInfo::new are stored in object and accessible to read
8. Removed ; from fetch_tag and fetch_ifd macro to get rid of compiler warnings

How to use:

 let raw = rawloader::decode_file(file).unwrap();
 if let Some(exif) = raw.exif {
     let iso = exif.get_uint(rawloader::Tag::ISOSpeed).unwrap();
     println!("ISO = {}", iso);
     let exp_time = exif.get_rational(rawloader::Tag::ExposureTime).unwrap();
     println!("Exposure time = {} seconds", exp_time);

     for tag in exif.get_tags() {
         if let Some(tag_str) = exif.to_string(tag) {
             println!("{:?} = {}", tag, tag_str);
         }
     }
 }