Improve handling of search fields.

Search fields are hard to work with since their labels are nil.  This
implements two changes to simplify that.

1. Passing `nil` into *any* accessibility label parameter should work
if the element's `accessibilityLabel` is also `nil`.
2. Add `waitForFirstResponderWithAccessibilityLabel:traits:`

This way, search field text can be accessed with a
`nil/UIAccessibilityTraitSearchField` combo.

Related to issue #257.
Made scroll view and table view methods use accessibilityIdentifier rather than accessibilityLabel.

These views shouldn't really receive accessibility labels.  With iOS
5.0, they should be using accessibility identifiers.
Update README.md

Removed reference to inactive OSX port.  Added details about supported OS versions, supporting issue #96.
Readded writing screenshots when tests fail.

Resolves issue #247.
Add system test step to capture on demand screenshots.

Resolves issue #145.
Update README.md

Added note about `:exclusive => true`
Merge pull request #259 from kif-framework/better-handle-search-fields

Improve handling of search fields.
Merge pull request #258 from kif-framework/accessibility-identifier

Made scroll view and table view methods use accessibilityIdentifier
Select the view from the highest window.

Per #196, this code didn't make a lot of sense.  It would end up just
taking the view from the `windows[0]`.  Here we actually stop when we
have a viable view.
Mention correct environment variable.
Merge pull request #264 from plu/master

Mention correct environment variable.
Merge pull request #257 from blakewatters/bugfix/update-searchbar-support-for-ios7

Rework `UISearchBarTextField` first responder detection to support iOS 7
Added test for entering return character.

Checks for behavior described in issue #262.
Implement KIF system actor steps supporting isolated controller test use cases

This pull request introduces a handful of methods onto `KIFSystemTestActor` that support using KIF to develop and test UIViewControllers in isolation.
The methods introduced provide a simple interface for instantiating a view controller, optionally yielding it to the test for configuration, and then
presenting it for subsequent interaction by the test case.

In general, this approach leverages KIF more as a functional testing harness than its traditional role in integration/acceptance tests. More specifically,
this approach is useful in a number of scenarios:

1. Doing view controller TDD as a separate concern from the rest of the app.
2. Developing view controllers that cannot yet be integrated into the main UI (perhaps due to missing design or external dependencies)
3. Testing view controllers in scenarios that are otherwise hard to recreate with the UI alone