Improved performance and creation of custom properties

[Tokazama]

Building on some previous discussion about improving performance and access to properties in ImageMeta, I've put together https://github.com/Tokazama/ImageProperties.jl. I'm presenting it as a package because it's more convenient than one long gist with comments and isn't ready to be a pull request.

I still have several things to do before I can break down what really needs community input, but I thought some people might be interested in it right now. Thoughts opinions and contributions are welcome.