Change for support of declared but not used types/xsd.Time/and data type conversion

Hi,
While using this great lib tool xsd-makepkg, I met some issues while generating go code from http://www.cablelabs.com/projects/metadata/specifications/xml_schemas/MD-SP-TITLE-I02.xsd
It imports another schema:
http://www.cablelabs.com/projects/metadata/specifications/xml_schemas/MD-SP-CORE-I02.xsd

The imported schema defined some types to be used in other schemas, while generating code package for it, the xsd-makepkg added those types but never rendered them into output. 

I tweaked a little bit and found a fix that worked for me as the commit 
https://github.com/lobatt/go-xsd/commit/788d3fa0cdabb079ca0b4ef18395a2ee5ad77b1b 
shows.

The other 2 commits are for 
* support of xsd.Time type https://github.com/lobatt/go-xsd/commit/6ae1c88d687aa6f73277aa76b165eac1d4578895
* handle data type conversion https://github.com/lobatt/go-xsd/commit/8070db0c90361503ed6ec8084720a33ce1eb7d7c

Which are both pretty simple but solved some problems so I included in this pull request.

It would be great if you can review the change and point out if I misunderstood anything.
Thanks!