Update the SDKs supported in CI.

It's not technically correct to say that 7.0 is no longer supported _at all_,
but we remove it from the list because we don't plan to test against it in CI.

We remove 5.1 because Travis no longer supports 5.1.
Add some missing escapes in `subliminal-test`'s documentation.
Use `instruments`' `-w` flag to enforce the simulator SDK & device type. (resolves #143)

This capability, added in Xcode 5.1, obviates the need to enable/disable
SDKs to force `instruments` to use a particular SDK for testing
(http://openradar.appspot.com/radar?id=3107401) and the need to
specify the `TARGETED_DEVICE_FAMILY` argument to force `instruments`
to use a particular device family for testing
(http://openradar.appspot.com/13607967).

The first components of the configurations respected by the `-w` flag
(the device types) are the same as the items listed in the iOS Simulator's
"Hardware -> Device" menu, so `subliminal-test` remains
backwards-compatible, despite its documentation changing.

Some references to iOS 5.1 have been updated in preparation for our
eventual dropping of 5.1 support (though we still do support it currently).
Deprecate the `--live` and `-login` password options of `subliminal-test`. (refs #176)

`instruments` may (and should) be "pre-authorized" using the instructions
at https://github.com/inkling/Subliminal/wiki/Continuous-Integration#faq.
Remove the `before_install` step from `.travis.yml`.

Travis initializes submodules for us as described here:
http://docs.travis-ci.com/user/build-configuration/#Git-Submodules
Don't try to unlink `appledoc` on Travis.

It's not (yet?) installed on the new 10.9 boxes.
Install `appledoc` from binary on Travis.

appledoc 2.1 uses GC and so cannot be compiled using Xcode 5.1,
so we download a pre-built version from the `gh-pages` branch
and manually install it.

The version was pre-built following the instructions [here][1] using
Xcode 5.0.2 and the `appledoc21` formula. I thought that maybe we
could install the resulting bottle [directly][2] but for some reason that
fails.

[1] https://github.com/Homebrew/homebrew/wiki/Bottles
[2] https://github.com/Homebrew/homebrew/wiki/Bottles#local-bottle-usage