Add support for some more TTNv3 fields

[jchonig]

Placement is now set on gateway if specified.

Description was being set to Name. Preserve this if only Description is specified. Otherwise set Name from Name and Description from Description.

Addressses #132

[terrillmoore]

I think it's going to be confusing to change things. My suggestion is to keep description as is and add 'extended-description' or similar.

(Alternately (and long-term better): explicitly version the yaml files (add a "schema-version" or similar, and make this a breaking change in teh schema and simply require name with v2 schema. Then someone can do a massive regular change on the input data to update to the newer schema if they want to take advantage of name/description.)

But just adding extended-description will be fine for now. As I said in my other comment, they're working on V4, and this is all subject to change anyway.