Use request.el

Use request.el (https://github.com/tkf/emacs-request) to avoid bugs in url.el.

This branch passes the test:
https://travis-ci.org/tkf/emacs-ipython-notebook/builds/3872279
But I need to test it in real use more before merge.  Probably I should add more unit/functional tests.

I think it is usable.  If you are courageous please use it and report bug if you find it.

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Check list before merge:

* Make sure that request.el is installable from MELPA
* Add request.el as a dependency in el-get recipe