Add an "int16" property type

[sgillies]

Resolves #1321

I might like to improve how the project enumerates its data types too.

[sgillies]

@mwtoews any thoughts on this?

[mwtoews]

Sure, some questions / remarks:

• Should int16 be described in this doc or specifically fiona.FIELD_TYPES_MAP? (also bool is missing here too, so I'm not entirely sure if this subdata type should be added there.)
• Currently fiona.schema.normalize_field_type("int16") returns int32; should it? The same function with bool returns bool.

[sgillies]

@mwtoews thank you! I'll check on the function, that looks like a bug.

[sgillies]

@mwtoews I'm going to address your concerns in a follow up PR.