Add setters and validators

[thomasmholder]

Right now, all header fields are validated only at encode time, which is bad because it obfuscates where something actually goes wrong. Validation should be done at construction and when fields are set via setters (which need to be created).

Additionally, the current validation logic uses ranges to verify that properties are legal, where we should probably be checking to see if they're a member of the appropriate enum. That logic can probably be broken out into static validater functions.