+[BFTask taskFromExecutor:withBlock] to easily create ad-hoc tasks

Created by: ide

This convenience class method makes it easier to create tasks from blocks without needing to write the code for a temporary BFTask or BFTaskCompletionSource. Usage:

[[BFTask taskFromExecutor:executor withBlock:id ^{
    return work_on_queue();
}] continueWithBlock:...];

As opposed to:

BFTaskCompletionSource *tcs = [BFTaskCompletionSource taskCompletionSource];
dispatch_async(queue, ^{
    [tcs setResult:work_on_queue()];
});
[tcs.task continueWithBlock:...];

The convenience method also handles exceptions the same way the rest of BFTasks does so that's another plus.

Test case is included.