Port over linting and test for typings

[timlindvall]

• Move typings from lib/ to types/.
• Add dtslint for validating types.
• Use grunt-bg-shell to call out to dtslint during build step.

Notes:

• dtslint is the tooling that was used previously to check types, and it's designed to validate typings, so it seemed like a good fit for this use case. Much of the directory structure in this PR mimics the instructions from https://github.com/Microsoft/dtslint.
• Couldn't find a grunt library to run dtslint, so I'm using grunt-bg-shell to proxy to a script in package.json that runs dtslint.
• Originally, I ran into an issue where dtslint was complaining about types in node_modules which seemed odd. But, after making a couple of tweaks to the typings test file, I'm not able to reproduce this anymore. I'm relying on the test run against this PR to tell me if this was a fluke or if the issue persists.

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[nknapp]

I think I would rather create a custom grunt task in the tasks-directory, than including another pacjage. Just use child_process.execFileSync to run dtslint.

But we can also merge your commit first, and then change the task. I'll look into it.

Thanks for supplying this.

[nknapp]

OK, never mind. I feared that grunt-bg-shell would be pulling tons of dependencies into tree. But it doesn't have any, so we might as well keep it.

[nknapp]

Released in 4.1.2