Switched from 'request' library to 'superagent'

[isabek]

Added superagent module
Changed request to superagent module.
Fixed tests

[jxm262]

Looks great, thanks! The only minor nitpick is in the tests 'xchange.test.js' you have the variable called 'superagent' which is different than the naming on the xchange.js implementation file (called 'request' there). I think you'd want to name the variable in this test file to 'request' to keep it the same. This also looks like how the superagent project has named the variable in their tests. Not a major thing though, just my initial thought.

Thanks again!

[isabek]

Oh. I will fix it