Remove various explicit deployment target settings

Removes explicit deployment target settings in various targets
in the Subliminal project so these targets will inherit the
setting from the project.

Subliminal is supposed to support iOS 5.1 and the project level
iOS deployment target setting is on 5.1, so now all the targets
will have that same deployment target.
Work around UIAutomation typeString bug in older versions of iOS

Implements a work around for a bug in UIAutomation that causes
the keyboard's typeString (JavaScript) method to fail after the
first character on older versions of iOS.  For iOS 6 and later
we can use typeString in a very natural way, but for older versions
we must break up the string to be typed and execute typeString
multiple times, once per character.
Mock objects are no longer used as app hook targets in unit tests. (refs #12)

Because ARC is incompatible to weak references to NSProxy-derived objects
in iOS 5 (http://stackoverflow.com/questions/9104544/how-can-i-get-ocmock-under-arc-to-stop-nilling-an-nsproxy-subclass-set-using-a-w).

We're still able to use mocks to verify that the hooks are called by partially mocking
real targets.
Make sure not to reference UIAccessibilityTraitHeader on iOS 5. (refs #12)

It's not available until 6.
Turn off auto-layout on two more XIBs. (refs #12)

These XIBs can't be unarchived on iOS 5, also we're not using
constraints anyway.
Attribute the proper bugs to UIAElement.isVisible on iOS 5. (refs #12)

It doesn't always return true for accessibility elements in table view cells,
as it does in iOS 6; but it does fail to update for a newly-appeared
table view cell child element.
Avoid using iOS 6-specific UITableView APIs in Subliminal's integration tests. (refs #12)
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.
Disable all testing against the 5.1 SDK when running on Travis. (refs #18)

They do not yet have that SDK installed: https://github.com/travis-ci/travis-ci/issues/1157.