Add python coverage module to third_party

Also add a top level Makefile to native_client_sdk which
excersises this.

Add exception to checklicenses.py since upstream does
not include license information in their source files.

BUG=435348,434551

Review URL: https://codereview.chromium.org/727003004

Cr-Commit-Position: refs/heads/master@{#305439}
54 files changed