Remove tests for futures

The tests for futures seem to open too many files on OS-X.

`python setup.py test` results in a "Too many open files" exception.  (This is on system python 2.7 on 64-bit Lion).  While upping the ulimit from the default of 256 to 1024 does seem to solve the problem, it does seem to be OS-X-specific -- lowering the ulimit on Linux to match OS-X does not seem to cause the problem there.

I don't really understand the futures code well enough to speculate what might be going on, or whether it's an interaction with something we're doing.  (Could it be specific to py.test, since the upstream tests were written to run with unittest?)

For this reason, and the fact that these tests take a significant portion of the runtime, I'd suggest we just remove them for now and revisit it when we're actually using futures in our own code (and at that point, we'll be testing in context anyway, which is probably better anyway).