[Polyfill] Make XMLHttpRequest compatible to whatwg-fetch

[wkh237]

Described in #69 , when replacing window.XMLHttpRequest react-native fetch becomes malfunction. Though it's still possible to use our fetch but we should still consider make XMLHttpRequest compatible to whatwg-fetch.

• Fetch replacement
• FileReader polyfill implementation
• Test cases
• Documentation

[wkh237]

It turns out if we are going to do this, we have to implement a FileReader.

W3C document

UPDATE

FileReader replacement does not work, we should implement Fetch replacement instead.

WHATWG-Fetch