Integration tests

I've added some structure to support integration tests.  Right now there are only a few, which exercise code similar to the ones in the `examples` directory, but the idea is to add more to cover different usage patterns that we intend to support.

Because it's a bit more hazardous to actually mount filesystems, I figured these needed to be a different target and not part of `make check`.

I tried to figure out how to actually use, rather than copy, the code in the `examples` dir for the integration tests, but I couldn't see how without adding integration test code to the examples themselves, which would then make the code files less useful as examples of normal use.

Right now, the "null-example" test does not work correctly under OS X--it passes its tests, but then fails to stop its background task and gets killed by the wrapper script after timing out.  This is due to #7.