Fix meta property declaration to be optional

[smithad15]

According to the comment immediately above the property, meta is supposed to be optional. This PR brings the type description in line with that sentiment.

[coveralls]

Coverage remained the same at 100.0% when pulling 6ea25b7 on smithad15:patch-1 into e55a736 on acdlite:master.

[JaKXz]

Thank you for the PR, but please see the discussion in #53.

[smithad15]

Understood, however, this does cause some confusion between the documentation and the type enforcement. If you are hesitant to change the type declarations while waiting for TS2.3, might I suggest changing the documentation to reflect the current limitations instead? Having a property described as optional, and then enforcing its inclusion only for TypeScript users is a little counterintuitive. Personally, I've had to implement my own type description in my projects in order to work around this. In the end, it's up to you. Thanks for the library!

[JaKXz]

@smithad15 thanks for the response, and sorry for my slow response. Please feel free to submit that documentation change that is relevant until TS 2.3, but for now I'm going to close this PR in favour of #53.

cc @wub, @unional