Use tiff tag types from tiff crate

[weiji14]

Adding the tiff crate as a dependency (per #7), and doing some slight refactoring to use the tiff:tags:Type enum instead of defining it in-house.

Note that this also adds partial support for IFD, LONG8, SLONG8 and IFD8 variant types (untested).

TODO:

• [x] Add tiff crate to Cargo.toml
• [x] Rename src/tiff.rs to src/geotiff.rs to reduce naming confusion
• [x] Use tiff::tags::Type enum instead of crate::lowlevel::TagType
• [x] Document in main README.md the transition to using the tiff crate