Add CLA check to travis build

Based on #5857 

Adds 2 new grunt tasks

```
# CLA check for pull request
grunt cla-check-pull --pull=NUMBER

# CLA check for user
grunt cla-check --user=GITHUB_USER
```

* Updates travis build to fail if the user has not signed the CLA.
* Only runs `grunt test` task if the build on travis-ci is a pull request build (vs. an ordinary commit build)
* Adds `tasks/cla-exceptions.json` for github users that are covered under non-standard CLAs

Assigning @dangoor. cc @busykai.