for RFC-7616 add SHA-256 and SHA-512

While working with some FIPS libraries that disabled MD5, we ran into a problem with our Requests based tools not being able to talk to our server. 

<img width="1440" alt="screen shot 2017-11-08 at 4 19 47 pm" src="https://user-images.githubusercontent.com/438131/32584052-c4a29c36-c4b3-11e7-8ca1-7601c60c60f2.png">

I haven't fully read the RFC-7616 SHA-256 and SHA-512 standards but this patch works.

Thanks for Requests!


# [Codecov](https://codecov.io/gh/requests/requests/pull/4378?src=pr&el=h1) Report
> Merging [#4378](https://codecov.io/gh/requests/requests/pull/4378?src=pr&el=desc) into [master](https://codecov.io/gh/requests/requests/commit/c3f2553114fdaf83b9c646a73987bbd2f13285e1?src=pr&el=desc) will **decrease** coverage by `0.51%`.
> The diff coverage is `0%`.

[![Impacted file tree graph](https://codecov.io/gh/requests/requests/pull/4378/graphs/tree.svg?token=mZGswIiXLa&width=650&height=150&src=pr)](https://codecov.io/gh/requests/requests/pull/4378?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4378      +/-   ##
==========================================
- Coverage   88.36%   87.85%   -0.52%     
==========================================
  Files          18       18              
  Lines        2055     2067      +12     
==========================================
  Hits         1816     1816              
- Misses        239      251      +12
```


| [Impacted Files](https://codecov.io/gh/requests/requests/pull/4378?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [requests/auth.py](https://codecov.io/gh/requests/requests/pull/4378?src=pr&el=tree#diff-cmVxdWVzdHMvYXV0aC5weQ==) | `80.45% <0%> (-5.96%)` | :arrow_down: |

------

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Could you add test cases for this, please?

Can do.

Status: I'm reaching out to Requests developers listed in "contributing" for some assistance adding tests. 

Status update: Twitter reply from @lukasaoz 
"  | Lukasaoz @linuxlizard I doubt httpbin supports these auth modes yet so you may need to begin by adding support there. Alternatively you can use code like test_lowlevel.py to force the correct responses.Nov 14, 2017, 12:01 PM
-- | --

I'm going to investigate that approach.


Status update: Twitter reply from @lukasaoz 
"  | Lukasaoz @linuxlizard I doubt httpbin supports these auth modes yet so you may need to begin by adding support there. Alternatively you can use code like test_lowlevel.py to force the correct responses.Nov 14, 2017, 12:01 PM
-- | --

I'm going to investigate that approach.


Created pull request for httpbin to add SHA-512 support.
https://github.com/kennethreitz/httpbin/pull/401


Thanksgiving over. httpbin patch accepted. Back to work on learning how to add tests to Requests.


Shutting down this pull request. Creating a new pull request with test code.