Add a missing license file

[kylef]

The MIT license states the following:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

Which renders the NPM package as unlicensed as it is packaged to NPM public repository without a LICENSE file and thus without the copyright and permission notice thus voiding the license.

[coveralls]

Coverage remained the same at 99.16% when pulling 5cc37dd on kylef/license into df5c331 on master.