TSLint should enforce license headers

[glennsarti]

I've contributed to a Java based GPL3 licensed product and it had a unit test to ensure that all files had the appropriate license header. Given the conversation in #1355 that all files (well TS at least) should have a standard license header it would make sense to add a TS Lint rule to enforce this.

Perhaps palantir/tslint#1441 gives guidance on how to enforce this rule.