Added capability to assign block results to keys for use inside examples...

Please let me know if this feature is of interest.  In order to dry-up code across multiple examples, I added an `assign` function which takes a block and assigns its results to a key.  The result can then be re-accessed from within any example, or nested example group/example.  This is along the lines of RSPec's `let`.  

````
describe(@"assign", ^{
    
  assign(@"foo", ^{ return @"bar"; });
    
  it(@"accesses assgined block return value", ^{
    NSString *foo = getAssign(@"foo");
    expect(foo).toEqual(@"bar");
  });

});
````

If this is of interest, I can add block result memoization.

FYI.  In order to get this working, and especially with nesting, I had to:
- Add a reference to a parent ExampleGroup to Example
- Add a reference to the currently executing example to the Spec (SPT_getCurrentExample didn't seem to work).

Thanks
b