Define tags as semi-private

[geospatial-jeff]

Made some improvements with #4 and #9 but I'm still not a huge fan.

I think it would be best to switch tags to semi-private attributes and expose important metadata through properties like here. At minimum we should have properties for the rasterio profile. A few reasons:

1. I don't think most users care about the metadata contained on each Tag object (or even care about all of the defined tags)
2. Keeping Tag defined on the IFD still resolves Define explicit IFD attributes for supported tags #9.
3. Properties of course will be more user friendly to use (ifd.width instead of ifd.ImageWidth.value).
4. Making Tag semi-private prevents confusion (ex. ifd.Compression vs ifd.compression is confusing)