show reviewed pull requests in the "sympy-bot list"

[certik]

check that the pull request was tested, and no new patch was added in, by looking at the comments and searching for "sympy-bot".

[asmeurer]

It's probably better to search for [sympy-bot](https://github.com/sympy/sympy-bot), or even Automatic review by [sympy-bot](https://github.com/sympy/sympy-bot), as people type "sympy-bot" in the comments all the time.

[asmeurer]

Actually, we probably need to keep track of reviewed SHA1 hashes in some database. The reason is that changed pull requests are not guaranteed to show commits after the latest review comment, particularly when a roll-back is force updated. Also, I never really checked how the commits show up in the API.

We could do this on the app engine, or we could have a simple git repo that everyone can push to that has all the relevant info there (and as I discussed on another issue, we could push up the html pages to gh-pages). I don't know how to use the app engine, so I would personally prefer the latter, unless you or someone who does know how to use the app engine wants to code it up.

[asmeurer]

Actually, I think just searching comments for SHA1 hashes should work. A database would work too (that's related to #63). Another way would be to implement #132.