upgrade tunnel-proxy to 0.4.0

  to avoid node 0.11.x issue (#779)
add some exposition to mpu example in README.md

This is re @mikeal's comment here:
https://github.com/mikeal/request/issues/345#issuecomment-13998419

This updates the mpu example in the README with some more comments and a callback to make the usage unmistakably clear for beginners and especially forgetful developers (e.g. @mikermcneil)
made the language clearer

don't want to give anyone any weird ideas
Merge pull request #810 from mikermcneil/patch-1

add some exposition to mpu example in README.md
Added support for HTTP_PROXY and HTTPS_PROXY environment variables, if the proxy option isn't already set.
Fix word consistency

`user` || `username` is used to describe the user field in the `auth` hash while `password` || `pass` was used to describe the password field. The order has been flipped for consistency: `pass` || `password`.
Merge pull request #850 from 0xNobody/master

Fix word consistency in readme
Merge pull request #809 from ksato9700/issue-779

upgrade tunnel-proxy to 0.4.0
Won't use HTTP(S)_PROXY env var if proxy explicitly set to null.
Add support for RFC 6750 Bearer Tokens
Add documentation about auth.bearer
Merge pull request #861 from phedny/master

Add support for RFC 6750 Bearer Tokens
Fix typo
Merge pull request #866 from dandv/patch-1

Fix typo
Clean code syntax in test-pipes.js

- fix not-defined 'emit'
- do not declare 'counter' as global
- remove trailing whitespaces
Add test for request.pipefilter

- check if request.pipefilter is invoked
- check ig request.piepfilter is called with correct arguments
Merge pull request #869 from tgohn/pipefilter_test

Pipefilter test
Fix typo in form example
Documented tough-cookie installation.
failing test