GH: closes #67

- Also do some switching away from time_now() back to time.time()
GH: ref #67

- Add note in CONTRIBUTING to prefer UTC and timestamps

- Fix things that broke when switching from date to datetime.

GH: closes #113

This allows people to run 'tox' without arguments on machines that don't
have all our target python versions installed.

GH: closes #115

- Uses regex
- Properly ignores files that don't look like YYYY-MM-DD*.txt
- Can search to arbitrary depth
- Returns results in a generator

It will remove files that it shouldn't. It fails to ignore files
from the current day and days in the future.

There's a known-to-fail test in another branch right now. If I didn't
know that and I was relying on Travis to catch the issue, I would not
be happy with the default level of output. (Specifically, the amount
of output in a list of file names is not enough IMHO. It it was a
different type of list, it might be okay)

There are times when a failure to build a circuit through a relay is
not the relay's fault. See the comment added in this commit for more
rationalization.

Even after thinking about it for a few minutes and triple checking I was
thinking in the correct direction, I still managed to get it backwards.

A failure to measure a relay is not necessarily that relay's fault. Add
(and use) code that allows different ResultError subtypes to reduce a
result's freshness by different amounts. This way, if a bad result was
not likely to be the relay's fault, it won't be penalized very much.

GH: closes #61

GH: closes #106

ReadTheDocs doesn't install sbws so we can't import the version.
So we have to essentially grep for it.

GH: closes #100

GH: closes #97