Added i18next TypeScript declarations source file + tests

Hi Boris,

i18next.d.ts file and test files as accepted by the library author Jan Mühlemann. See pull request: https://github.com/jamuhl/i18next/pull/64

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.

I hope this can be merged.

Kind regards,

Maarten Docter