i18next TypeScript declaration source file

Hi Jan,

I created an 'i18next.d.ts' TypeScript declaration source file and added test files.
Can you please check the file if it really matches your public API. I did my best to find all methods and properties etc. but maybe I missed something.

The test files are included because I will also create a pull request
for the DefinitelyTyped repo by Boris Yankov
(https://github.com/borisyankov/DefinitelyTyped).

The jQuery / Mocha and Sinon *.d.ts files aren't my work and some seem
to be incomplete, but are only included because the original test page
(http://i18next.com/pages/test.html) uses them and they do the job.