Minor fixes to API documentation.

[skip ci]
Update the README to refer to XCTest rather than SenTest.

Sign of the times. And SenTest(ingKit) is part of OCUnit anyway.

[skip ci]
Re-enable processing of `SLLogger` documentation.

`SLLogger` stopped being processed when it was moved to its own
top-level directory (when the `SLLogger` framework was created).

[skip ci]
Wrap the contributing guidelines to roughly 80 characters.

[skip ci]
Mention that Appledoc installs the docset in the contributing guidelines.

[skip ci]
Adjust which headers are public/private.

SLGeometry is probably useful enough to people making their own SLElement
subclasses to make public-requiring-special-import;
`UIScrollView+SLProgrammaticScrolling` is private and should be respected as such
when Cocoapods installs.
Suppress a warning when Subliminal is built with assertions disabled.

As will occur when Cocoapods installs, due to
https://github.com/CocoaPods/Xcodeproj/pull/53.
Merge pull request #188 from inkling/jeff/update_documentation

Update the API documentation.