Restructure git repository

Our current layout is a bit redundant, in that most of our artifacts are in yang/ directory and most start with yang- prefix.

As our scope is pretty much nailed down and we know the general structure of our deliverables, we should have a few top-level directories into the content of yang/ is distributed:

• model/, hosting yang-model-api and all related API/SPI/util artifacts
• data/, hosting yang-data-api and related artifacts (including yang-data-impl)
• codec/, hosting yang-data-codec-*
• parser/, hosting yang-parser* and *parser-support artifacts
• plugin/, hosting maven plugin and plugin-generator-api
• tools/, hosting yang-validation-tool

Also, yang-common-* should be moved to common/ .