Slightly relax the `SLTerminalTest` timeouts.

I think that the existing variability only accounted for the evaluation of the condition,
and an additional evaluation delay is needed for the wait function as a whole. That would
tally with this Travis failure: https://travis-ci.org/inkling/Subliminal/builds/10175832#L1863.
Clarify the documentation of `SLWaitUntilTrue`.
The tests of `SLWaitUntilTrue` now all take into account the delay in that macro retrying,
as appropriate. (refs #53)
All tests concerning timeouts now assert that Subliminal waits for *at least* the specified
timeout. (fixes #53)

While there is variability in timeouts, that variability should always be positive
--Subliminal should never stop waiting while there is time remaining.
Properly account for the delay inherent in asking UIAutomation to look up an element's value.
Merge pull request #59 from inkling/jeff/53_more_timeout_adjustments

More fixes to tests using timeouts.