Read Short as u16, SignedShort as i16

[weiji14]

Previously, Short values were read incorrectly as u32 instead of u16, and SignedShort values were read incorrectly as i32 instead of i16.

This PR does the following:

• Add TiffFormatError::ShortExpected enum variant
• Correctly cast Type::SHORT as u16 values, and support casting Short to other unsigned types via the into_u32_vec and into_u64_vec methods.
• Correctly cast Type::SSHORT as i16 values

Fixes #204