Add Typescript Declarations

[naerbnic]

Added index.d.ts that contains the typings (as I have read them) from the rest of the library.

[naerbnic]

If you see any problems with the typings given here, or if you have any idea of the right way to test this, I'm happy to hear it.

One thing I was thinking is rewriting one or more examples to use Typescript instead, and check that the usage of the library matches them.

[coveralls]

Coverage remained the same at 98.66% when pulling 4f942b0 on naerbnic:add_typescript_declarations into 9678b7d on clarkie:master.

[jkav77]

@naerbnic If you write some examples for the README.md then I think this is good to merge, right?

[cdhowie]

I don't think tests are required. However, these definitions would need updating any time we make changes to our API. Do we have the manpower+discipline+expertise to keep these definitions up to date? @clarkie @dangerginger

[stephentuso]

There is a declaration file and example usage available in DefinitelyTyped (@types/dynogels), but it is definitely preferable for types to be maintained here alongside the library. I've been using those without issue, I haven't looked at the differences in depth but there are at least a few things that could be added to this one, or might be best to just copy the other one