* Prefer third-party plugin development in plugins instead of kibana-extra
* Fix failing recursive directory creation and removal
* Add new built version of kbn-pm
In order to make the license that applies to each file as clear as possible, and to be consistent with elasticsearch, we are adding Apache 2.0 license headers to the top of each file.
Existence of this header is enforced by eslint and tslint and missing headers were automatically added in the last commit by running:
```
node scripts/eslint --fix && node scripts/tslint --fix
```
