Changed Wikihouse Plugin to be compatible with Sketchup Extensions.

Main additions:
* `make` or `make build` now generates `wikihouse_extension.rbz` 
* `make release` *should* upload following to Amazon S3 bucket:
    * `wikihouse_extension.rbz` and `wikihouse_extension-{git-rev}.rbz`
* Included `.gitignore` for .rbz files
* Renamed all instances of plugin to extension for clarity. 
* Updated install instruction in README