Re-introduced a `captured` property to check whether an argument was captured on KWCaptureSpy

Squashed commit of the following:

commit 4a8bb47c2951d0e0b600922c8a0cb5db044e0b31
Merge: a9f7c07 803825e
Author: Eric-Paul Lecluse <e@epologee.com>
Date:   Tue Nov 12 13:10:29 2013 +0100

    Merge remote-tracking branch 'allending/master'

commit a9f7c073a1b93773507f928447c7d1b65ee47874
Merge: 46e0689 d95c8c4
Author: Eric-Paul Lecluse <e@epologee.com>
Date:   Thu Oct 31 15:17:10 2013 +0100

    Merge remote-tracking branch 'origin/master'

commit 46e0689169275b0b1e9671b6d09e6a52a8db6248
Author: Eric-Paul Lecluse <e@epologee.com>
Date:   Thu Oct 31 15:14:04 2013 +0100

    Added public property for checking the capture.

commit 6656e52ee9e3364a31b57b086627ff16bef822c8
Author: Eric-Paul Lecluse <e@epologee.com>
Date:   Mon Oct 21 14:53:36 2013 +0200

    Removed dependency on argument being nil for checking whether an argument was captured.

commit d724f78c3f391dac9fcde3dfe03553741874ad97
Author: Eric-Paul Lecluse <e@epologee.com>
Date:   Mon Oct 21 14:20:54 2013 +0200

    Re-introduced property on KWCaptureSpy to poll whether the argument is captured already.

commit c1c915ec88abbc44a9c39234f77a9e228cb0f2b7
Author: Eric-Paul Lecluse <e@epologee.com>
Date:   Mon Oct 21 11:12:18 2013 +0200

    Added a flag to prevent KWCaptureSpy from raising an error when the argument is nil.

    This makes it possible to use [[expectFutureValue(spy.argument) should] beNonNil] in asynchronous captures.

commit d95c8c46f93200eda5ce7202f958de3f18c23b92
Author: Eric-Paul Lecluse <e@epologee.com>
Date:   Thu Oct 31 15:14:04 2013 +0100

    Added public property for checking the capture.

commit 4a110f8ca09649ff9f18b3295c6aa6a8449b5639
Author: Eric-Paul Lecluse <e@epologee.com>
Date:   Mon Oct 21 14:53:36 2013 +0200

    Removed dependency on argument being nil for checking whether an argument was captured.

commit 66c4c66c9fd4ee6b19d9149ffd546d9a1d39387f
Author: Eric-Paul Lecluse <e@epologee.com>
Date:   Mon Oct 21 14:20:54 2013 +0200

    Re-introduced property on KWCaptureSpy to poll whether the argument is captured already.

commit 34d42917b1bd3c8cd207a8dd3badb95892b3a268
Author: Eric-Paul Lecluse <e@epologee.com>
Date:   Mon Oct 21 11:12:18 2013 +0200

    Added a flag to prevent KWCaptureSpy from raising an error when the argument is nil.

    This makes it possible to use [[expectFutureValue(spy.argument) should] beNonNil] in asynchronous captures.