Refactor `-[SLElementMatchingTest testMatchingTableViewHeaderChildElements]`' view.

To use a custom view.
Instances of `SLElement` can be directed to double-check their validity using UIAutomation.

To diagnose bugs in `SLAccessibilityPath` and/or `NSObject (SLAccessibilityHierarchy)`,
where Subliminal believes an element to be valid but incorrectly identifies it to UIAutomation,
as it is not guaranteed that UIAutomation will raise an exception if an accessibility path
is invalid.

This setting is disabled by default because it is more likely that there is a bug in
a particular test than in UIAutomation and because enabling it will negatively
affect the performance of the tests (by requiring an extra call to UIAutomation).
Clarify `SLAlert`'s documentation.
Subliminal now reloads the accessibility hierarchy as necessary while matching.

In certain circumstances, accessibility elements may become "stale", in that they match
views which are no longer in the hierarchy. When these elements are queried for their
accessibility information, their container will reload _all_ its child elements.
This could result in Subliminal finding a path to an element only to have the process
of finding that path invalidate the path! We now preemptively reload the accessibility
hierarchy while retrieving accessibility elements to ensure that accessibility paths remain
valid.
Merge pull request #74 from inkling/jeff/reload_accessibility_hierarchy

Subliminal reloads the accessibility hierarchy as necessary while matching.