Work around scroll views not being tappable when using an iPad running iOS 5.x
as best possible. (refs #12)

When using an iPad simulator or device running iOS 5.x, UIAutomation always reports
scroll views as not visible, and hence not tappable. Tapping a scroll view
on such a platform will fail, though dragging succeeds (despite error messages
being logged). Accordingly, Subliminal no longer enforces tappability if the target
element is a scroll view and the tests are running on an iPad simulator or device
running iOS 5_x. This will let dragging succeed, at the least.

The bug has been documented on `-[SLUIAElement isTappable]` and `-tap`,
and tests have been added to confirm that Subliminal allows user interaction
to proceed even if it may fail. A "Known Issues" section has also been added to the
README.
Increase the startup timeout. (refs #12)

With the simulator running iOS 5, I'm seeing Instruments take a few seconds
to actually start running SLTerminal.js. Strangely, this delay doesn't take place
on the first run of Instruments--only if you make changes to the tests and then
relaunch Instruments.

Anyway, we now give Instruments a few more seconds, on both platforms,
before we decide that SLTerminal/SLTerminal.js have failed and abort the tests.
Execute the character-wise typing loop entirely within JavaScript. (refs #12)

For improved performance, both in terms of not needing to create substrings
and not needing to send multiple commands to the terminal.
Move the SLTextField test cases' text fields up above the keyboard.

This is just an aesthetic tweak.
`rake test:unit` now executes Subliminal's unit tests against both iOS 5.1 and 6.1. (refs #12)
Factor `subliminal-test` build command into function.

To avoid recreating the command in the multiple places it is used.
`subliminal-test` now optionally tests against a particular simulator SDK. (refs #12)

This entails setting the SDK used by the iOS Simulator to the desired version,
then "hiding" the other SDKs, by renaming their `SDKSettings` plists, to force
instruments to use the current setting--it defaults to using the latest installed
(http://openradar.appspot.com/radar?id=3107401).
Subliminal now runs its integration tests on both iOS 5.1 and 6.1. (resolves #12)

Now that Subliminal fully supports iOS 5, the Requirements, Continuous Integration,
and FAQ sections of the README have been updated with information about testing
across different SDKs.