implement timed backoff on server failures

This is based on raven-python's implementation, with some tweaks for
ruby idioms.

With this commit, if we don't send, we only log the event id. I'd like
to extend this to log the entire event, along with also logging the
entire event when a sending failure occurs.
log event message on send failure or skip
add a little breathing room to the timing in the specs

Using timecop to freeze at the *precise* moment where the condition
changes can cause problems due to rounding error.
style tweak: remove parens from method calls with no args
Merge pull request #267 from codekitchen/failure-backoff

Failure backoff

Close #30