Improved test specificity.

Also added WordPress file usage, but CORS might cause it to fail, I
can't test that locally.
Added Node.js module compatibility.

Used UMD returnExports.js example.
Added test for corrupt/incomplete files.
Node.js apparently needs a library for Promises.
Testing improvements.

* The file reading function also gets tested now.
* Added the first valid MO file parse test.
* Get the mo files to test with using wget.
Node doesn't have an XMLHttpRequest object.
Fixed TextDecoder not getting defined for Node.