Re-license to MIT + Apache 2

Following https://github.com/ipfs/go-ipfs/issues/6302 path which determined that dual-licensing as MIT and Apache 2 is a best practice.

> This has two major benefits:
>
> - There are concerns in the open source community about whether the MIT license leaves users vulnerable to patent infringement claims. We think the pure legal risk is small, but the way the open source community interacts with our project is really important. It makes sense to pick the license that makes the largest number of people comfortable.
> - There's now no reason to adopt a separate DCO, since the Apache-2 license grant addresses the same issue.
>
> Why use a dual license, instead of just Apache-2? The Apache-2 license is incompatible with the GPLv2 license, which includes things like the Linux kernel. With a dual license, GPLv2 projects can just use the MIT license instead. Our goal is to make our software available to as many projects as possible, so we'd rather adopt a licensing scheme that doesn't exclude anyone.

What we need to do:

I have updated the licenses in #381 , the next step is to get an explicit OK from our current and past contributors to consent to the relicensing. To keep track of things, below is a contributor sign-off list. Contributors can either check the box next to their github handle, or comment on this issue thread with the following text:

> I license past and future contributions under the dual MIT/Apache-2.0 license, allowing licensees to choose either at their option.

- [x] @carsonfarmer
- [x] @andrewxhill
- [ ] @pcowgill

Review the license updates referenced in #381 and the contributor sign-off checklist in this issue. Start by confirming the required consent wording and the remaining contributor entry, then record explicit approval; done means all current and past contributors are accounted for and the dual-license consent is documented.

Written by the indexing model from the issue text.