Initial unit test coverage

:warning: Not ready to merge – opening for initial feedback :warning: 

I wanted to open this pull request sooner rather than later in order to get some feedback on the direction. As discussed in splinesoft/SSDataSources#2, the goal is to add some basic unit test coverage of SSDataSources.

The largest change so far has been infrastructural, namely, in commit 3793fc5 where I consolidated the two example Xcode projects into a single project. This is the pattern of the [CocoaPods example library](https://github.com/CocoaPods/CocoaPodsExampleLibrary), and it allows us to keep one project as the host of the unit tests.

Other than that, I have added [Expecta](https://github.com/petejkim/expecta) and [OCMock](http://ocmock.org/) as dependencies to the example Xcode project, and flushed out some tests for `SSBaseDataSource`.

If this all looks to be inline with the goals and conventions of SSDataSources, I'll proceed with expanding the test coverage.