Update the example app instructions for iOS 7.

Subliminal supports iOS 7 simulators too.

[skip ci]
Clarify that users should run the example app using the  "Integration Tests" scheme.
(refs #137)

Rather than the "Subliminal Integration Tests" scheme, if they see that because
Xcode autocreated it. That scheme isn't appropriate and can't be run anyway
unless the user initializes the OCMock submodule, on which those tests
depend.

[skip ci]
Clarify that Subliminal's xcconfig file should be added to the host project without copying.
(refs #137)

[skip ci]
Merge pull request #139 from inkling/jeff/example_clarification

Clarify the example and installation instructions.
Subliminal's tests now observe a pre-existing value of DEVELOPER_DIR.

Allowing e.g. Xcode 5.0.2 to be used alongside a "default" install of Xcode 5.1.
Merge pull request #140 from inkling/jeff/respect_DEVELOPER_DIR

Subliminal's tests now observe a pre-existing value of DEVELOPER_DIR.