added i18next TypeScript declarations source file + tests

Added the 'i18next.d.ts' file and test files.

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.