return result from callback

allow passing an agent through to node http requests, closes #68

closes #87

for more details see https://remysharp.com/2011/04/21/getting-cors-working

returning the result of the callback allows you to chain the promise
like you would expect

closes #56

closes #57

Fixing dependencies version

closes #56

closes #57

closes #89

Fixing arrays in get params

In browsers check for XMLHttpRequest, not window

Test for ActiveXObject

Fixing isArrayLike to allow length as a param. Added test

In nodejs testing environment it's possible to use https://github.com/tmpvar/jsdom library to define a window object, but you still want to use node http adapter. Due to those diverse testing environmnents, I propose to test for XMLHttpRequest directly, because window global object is not a sure sign of a browser environment anymore.