[artoolkitNFT.api.js] convert ajax function to FS.readFile instead of using XMLHttpRequest.

[kalwalt]

As stated here in this #128 (comment) we should try to make this change to solve the issue.

[kalwalt]

For the node only version of the lib i will use FS.readFile but as i'm doing in this PR #140 where i'm developing separate libs for Node.js. Anyway i would try to convert in artoolkitNFT.api.js the XMLHttpRequest with other alternatives if possible.