Add NOTICE file for Beats (elastic#3334)

* Python script to generate the NOTICE file

The script receives a directory and searches for all the all vendor subdirectories in order to extract copyright
information about the 3rd party libraries that are used. The NOTICE file is
generated in the current directory.

For each 3rd party library, the following rules are used:
- If the library has apache license, and if NOTICE file is present, include that text in  the NOTICE file.
- If the library has MIT, ISC or BSD license, include the whole copyright block in NOTICE file (meaning copyright + license)

* Accept also 3rd party libraries with less than 3 directories

Makefile

@@ -113,3 +113,9 @@ upload-package:
 .PHONY: release-upload
 upload-release:
 	aws s3 cp --recursive --acl public-read build/upload s3://download.elasticsearch.org/beats/
+
+.PHONY: notice
+notice: 
+	python dev-tools/generate_notice.py .
+
+